r600/sb: Fix build failure with non-standard libdrm installation prefix

Just like radeon/uvd, r600/sb fails to find the libdrm includes.

Signed-off-by: Lauri Kasanen <cand@gmx.com>

src/gallium/drivers/r600/Makefile.am

@@ -15,6 +15,7 @@ AM_CXXFLAGS = \
 	-I$(top_srcdir)/include \
 	-I$(top_srcdir)/src/gallium/include \
 	-I$(top_srcdir)/src/gallium/auxiliary \
+	$(RADEON_CFLAGS) \
 	$(DEFINES)
 
 libr600_la_SOURCES = \