automake: loader: honour the XCB_DRI3 cflags

Without this the compilation will fail, as the headers are installed in
a non-default location.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/loader/Makefile.am

@@ -30,6 +30,7 @@ AM_CPPFLAGS = \
 	-I$(top_srcdir)/include \
 	-I$(top_srcdir)/src \
 	$(VISIBILITY_CFLAGS) \
+	$(XCB_DRI3_CFLAGS) \
 	$(LIBDRM_CFLAGS) \
 	$(LIBUDEV_CFLAGS)