configure: Don't use cross_prefix when invoking pkg-config

configure: Don't use cross_prefix when invoking pkg-config

pkg-config doesn't need to have a cross prefixed version. It can be
pointed at alternate sysroots with environment variables like
PKG_CONFIG_SYSROOT_DIR. The configure script was already inconsistent
with using `pkg-config` in some places and `${cross_prefix}pkg-config`
in others. Make check_min_lib_version() and the gtk checks consistent
with the rest by dropping the `${cross_prefix}` usage with pkg-config.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Link: https://lore.kernel.org/r/20240123202636.179467-1-chris.packham@alliedtelesis.co.nz
Signed-off-by: Jens Axboe <axboe@kernel.dk>