Android.mk: Add -D_GNU_SOURCE to common_cflags

Other build scripts define -D_GNU_SOURCE. The Android build
system should too.

Fixes the following warning:

external/selinux/libsepol/cil/src/cil_mem.c:109:7: warning: implicit
  declaration of function 'vasprintf' is invalid in C99
  [-Wimplicit-function-declaration]
1 file changed