API: libcrmcommon: Add new PCMK_META_PRIORITY string constant

The old XML_RSC_ATTR_PRIORITY was removed as unused by commit f0ff729,
because common_unpack() (now pe__unpack_resource()) incorrectly used
XML_CIB_ATTR_PRIORITY.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>