xmlpool: make sure we ship options.h

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>