dri3: Add missing libdrm dependency in Makefile.am

Fixes: 0ce93e5b

 "dri3: Include dix-config.h instead of
                     xorg-config.h"
Signed-off-by: Dor Askayo <dor.askayo@gmail.com>

dri3/Makefile.am

 noinst_LTLIBRARIES = libdri3.la
 AM_CFLAGS = \
-	@DIX_CFLAGS@
+	@DIX_CFLAGS@ \
+	@LIBDRM_CFLAGS@
 
 libdri3_la_SOURCES = \
 	dri3.h \