pppd: Fix pppol2tp-android.so build

This patch enables USING_UAPI for pppol2tp-android plugin and fixes
related build errors.

1. Enable USING_UAPI for pppol2tp-android build. When trying to build
pppol2tp-android.so as is (in current pppd codebase) we will see build
errors like this one:

    bionic/libc/kernel/uapi/linux/ppp_defs.h:59:6:
        error: redefinition of 'NPmode'
    external/ppp/pppd/include/net/ppp_defs.h:142:6:
        note: previous definition is here

This is basically because USING_API option is not defined when building
pppol2tp-android.c, though it should be. Enabling that option fixes the
build. Detailed explanation is below.

Unlike the upstream pppd project, we don't keep include/linux/* headers
inside of pppd codebase. Instead, we define USING_UAPI when such headers
(user-space API [1]) are used, providing missing functionality in
ppp_defs.h file. From pppol2tp-android.c file it's obvious that UAPI is
used there (as linux/* files are included there). So we need to enable
USING_UAPI definition. Whether UAPI can be used or not is checked by
inspecting PPP_ADDRESS definition (if defined -- then we are actually
using UAPI, so USING_UAPI is defined). But for PPP_ADDRESS to be defined
we need to include <linux/ppp_defs.h> first (just like it's done in
sys-linux.c). So let's do that for all Linux systems, before actually
checking PPP_ADDRESS.

2. If we try to build pppol2tp-android plugin with current pppd
codebase, we will see next build errors (when USING_UAPI is enabled):

    external/ppp/pppd/include/net/ppp_defs.h:142:17:
        error: field has incomplete type 'struct ifreq'
    external/ppp/pppd/include/net/ppp_defs.h:143:21:
        error: field has incomplete type 'struct ppp_stats'

Let's include net/if.h for struct ifreq, and linux/ppp_defs.h for
struct ppp_stats.

[1] https://lwn.net/Articles/507794/

Change-Id: I3338a78b5579229ebe3dbd908a9a9f37aa63e042
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/pppd/include/net/ppp_defs.h b/pppd/include/net/ppp_defs.h
index a21afdb..9522290 100644
--- a/pppd/include/net/ppp_defs.h
+++ b/pppd/include/net/ppp_defs.h
@@ -38,6 +38,10 @@
 #ifndef _PPP_DEFS_H_
 #define _PPP_DEFS_H_
 
+#if defined(__linux__)
+#include <linux/ppp_defs.h>
+#endif
+
 #if defined(PPP_ADDRESS)
 #define USING_UAPI
 #endif
@@ -131,6 +135,10 @@ typedef u_int32_t	ext_accm[8];
  * What to do with network protocol (NP) packets.
  */
 #if defined(USING_UAPI)
+
+/* For struct ifreq */
+#include <net/if.h>
+
 /* This stuff isn't in uapi. TODO: is there a newer pppd that doesn't use this? */
 #define ifr__name b.ifr_ifrn.ifrn_name
 #define stats_ptr b.ifr_ifru.ifru_data