build: fix out-of-tree builds in gallium/auxiliary

build: fix out-of-tree builds in gallium/auxiliary

The rules were writing files to e.g. util/u_indices_gen.py, but in an
out-of-tree build this directory doesn't exist in the build directory.  So,
create the directories just in case.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>