cpg: Fix signedness of munmap result

munmap is signed and return value of zcb_free is also signed so storing
munmap result into unsigned value doesn't make sense.

Similar issue is fixed in zcb_by_addr_free function.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>