configure: move cmocka library detection

configure: move cmocka library detection

`PKG_CONFIG` variable needs to be set for `PKG_CHECK_MODULES` to
succeed, but this wasn't happening in Fedora because the first
appearance of `PKG_CHECK_MODULES` was conditionally skipped because this
distribution is compiled without `libbsd` support. Thus, moving the
cmocka library detection before libbsd fixes the problem.

Suggested-by: Lukas Slebodnik <lslebodn@fedoraproject.org>
Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>