mesa: rename format_info.c to format_info.h

mesa: rename format_info.c to format_info.h

The file is auto-generated, and #included by formats.c. Let's rename it
to reflect the latter. This will also help up fix the dependency
tracking by adding it to the _SOURCES variable, without the side effect
of it being compiled (twice).

v2: Update .gitignore to reflect the rename.

Cc: "10.4, 10.5" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>