i965: Add $(WNO_OVERRIDE_INIT) to AM_CFLAGS

brw_surface_formats.c and genX_blorp_exec.c do this a lot, causing lots
of warnings from clang.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

src/mesa/drivers/dri/i965/Makefile.am

@@ -41,6 +41,7 @@ AM_CFLAGS = \
 	-I$(top_srcdir)/include/drm-uapi \
 	$(DEFINES) \
 	$(VISIBILITY_CFLAGS) \
+	$(WNO_OVERRIDE_INIT) \
 	$(LIBDRM_CFLAGS) \
 	$(VALGRIND_CFLAGS) \
 	-msse2