compositor-android: fix build flags

When adding a C++ file to the build, I forgot to set CXXFLAGS for it,
triggering the error:

android-framebuffer.h:26:21: fatal error: EGL/egl.h: No such file or
directory

I never hit this, because I have EGL headers installed also in my
system, rather than only in $prefix.

Fix this by setting the CXXFLAGS for the android backend.

Reported-by: Scott Moreau <oreaus@gmail.com>
Reported-by: Tiago Vignatti <tiago.vignatti@intel.com>
Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

src/Makefile.am

@@ -130,6 +130,7 @@ android_backend = android-backend.la
 android_backend_la_LDFLAGS = -module -avoid-version
 android_backend_la_LIBADD = $(COMPOSITOR_LIBS)
 android_backend_la_CFLAGS = $(GCC_CFLAGS) $(COMPOSITOR_CFLAGS)
+android_backend_la_CXXFLAGS = $(GCC_CFLAGS) $(COMPOSITOR_CFLAGS)
 android_backend_la_SOURCES = \
 	compositor-android.c \
 	android-framebuffer.cpp \