API: libcrmcommon: Deprecate crm_copy_xml_element()

We also de-inline it to prevent test-headers from complaining about
undefined reference to crm_element_value(). I'd like to avoid circular
includes in xml_element_compat.h, and this way seems better than
re-declaring crm_element_value().

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