Update mesa/drivers/x11/Makefile.am for xm_image.h removal

Update mesa/drivers/x11/Makefile.am for xm_image.h removal

Commit 6c6803f28de0d4cb6937fcd95a47aa81da31fd78 removed xm_image.[ch], and removed
xm_image.c, but not xm_image.h from the Makefile, this was subsequently carried over
into Makefile.am

Remove xm_image.h from Makfile.am.  This allows 'make dist' to succeed, even if it
doesn't do anything useful

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>