meson: fix getting cflags from pkg-config

meson: fix getting cflags from pkg-config

get_pkgconfig_variable('cflags') always returns an empty list, it's a
function for getting *custom* variables.

Meson does not yet support asking for cflags, so explicitly invoke
pkg-config for now.

Fixes: 68076b87474e ("meson: build gallium vdpau state tracker")
Fixes: a817af8a89eb ("meson: build gallium xvmc state tracker")
Fixes: 1d36dc674d52 ("meson: build gallium omx state tracker")
Fixes: 5a785d51a6d6 ("meson: build gallium va state tracker")
Reviewed-by: Dylan Baker <dylan.c.baker@intel.com>