configure: use use_pkg_config() instead of check_pkg_config() for libsmbclient

This ensures that the CFLAGS and LDFLAGS are actually applied.
Fixes an incorrect change introduced with the clean-up in commit
cfcaf6b38e39ed6e788abb1a5a44f23660dce2f6.

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
