loader: Include include/drm-uapi in the autotools build

We're already including it in the meson build.  This fixes build issues
on systems which have a drm_fourcc.h that doesn't have modifiers.

Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/loader/Makefile.am

@@ -30,6 +30,7 @@ AM_CPPFLAGS = \
 	-DUSE_DRICONF \
 	$(DEFINES) \
 	-I$(top_srcdir)/include \
+	-I$(top_srcdir)/include/drm-uapi \
 	-I$(top_srcdir)/src \
 	$(VISIBILITY_CFLAGS) \
 	$(XCB_DRI3_CFLAGS) \