util/meson: Getting mesa util core to be self contained

util/meson: Getting mesa util core to be self contained

Guard util to be standalone by move subdir('util') just behind subdir('android_stub')

texcompress_rgtc_tmp.h is not a standard c header, do not treat it as source

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19522>