xmlpool: make sure we ship options.h

The header is included in ../xmlpool.h. With the latter of which used
directly in a number of places in mesa.
Note that we can also add it (alongside t_option.h) to noinst_HEADERS,
but neither solution fixes the issue that brough us here - namely:
Do not regenerate the headers, if it already exists.

Cc: "10.5" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/drivers/dri/common/xmlpool/Makefile.am

@@ -52,7 +52,7 @@ POT=xmlpool.pot
 
 .PHONY: all clean pot po mo
 
-EXTRA_DIST = gen_xmlpool.py t_options.h $(POS) SConscript
+EXTRA_DIST = gen_xmlpool.py options.h t_options.h $(POS) SConscript
 BUILT_SOURCES = options.h
 CLEANFILES = $(MOS) options.h