Update ext/gmp/tests/gh16870.phpt

Add XFAIL header for CI

Co-authored-by: Gina Peter Banyard <[email protected]>

Author: GaryAllan

ext/gmp/tests/gh16870.phpt

@@ -2,6 +2,8 @@
 Bug GH-16870 (Overly restrictive gmp_pow overflow checks)
 --EXTENSIONS--
 gmp
+--XFAIL--
+Currently fails after the introduction of overflow warning
 --FILE--
 <?php
 echo '64^11 = 0x', gmp_strval(gmp_pow(64, 11), 16), "\n";