src/Makefile.am: forward libvlccore CPPFLAGS

Some flags were defined in configure.ac but were not used in the
Makefile.am. No issue raised from this oversight since the test was
actually always disabled.

src/Makefile.am

@@ -405,6 +405,8 @@ libvlccore_la_LIBADD = $(LIBS_libvlccore) \
 	$(LTLIBINTL) $(LTLIBICONV) \
 	$(IDN_LIBS) $(SOCKET_LIBS) $(ATOMIC_LIB) $(LIBRT) $(LIBDL) $(LIBM)
 
+libvlccore_la_CPPFLAGS = $(AM_CPPFLAGS) $(CPPFLAGS_libvlccore)
+
 if HAVE_WIN32
 libvlccore_la_SOURCES += \
 	win32/error.c \