Update NEWS for pcntl fix [skip ci]

Author: NattyNarwhal

NEWS

@@ -13,6 +13,10 @@ PHP                                                                        NEWS
   . Fixed bug GH-21052 (Preloaded constant erroneously propagated to file-cached
     script). (ilutov)
 
+- PCNTL:
+  . Fixed signal handler installation on AIX by bumping the storage size of the
+    num_signals global. (Calvin Buckley)
+
 - PCRE:
   . Fixed re-entrancy issue on php_pcre_match_impl, php_pcre_replace_impl,
     php_pcre_split_impl, and php_pcre_grep_impl. (David Carlier)