glapi/glx: Treat xserver generated targets as .PHONY

Meaning, always rebuild them when asked instead of bothering to look at
timestamps (and then wondering why nothing happened when you said make).

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>

src/mapi/glapi/gen/Makefile.am

@@ -100,6 +100,8 @@ XORG_OUTPUTS = \
 	$(XORG_GLX_DIR)/indirect_size_get.h \
 	$(XORG_GLX_DIR)/indirect_table.c
 
+.PHONY: $(XORG_OUTPUTS)
+
 ######################################################################
 
 API_XML = \