Include <xlocale.h> for older macOS.

Commit 35eeea62 forgot to include <xlocale.h> when using locale_t
(which didn't seem to be required on newer Apple SDK as used by CI,
hence mistake).  Let's see if this fixes build farm animals longfin and
sifika.

src/port/chklocale.c

@@ -23,6 +23,10 @@
 #include <langinfo.h>
 #endif
 
+#ifdef LOCALE_T_IN_XLOCALE_H
+#include <xlocale.h>
+#endif
+
 #include "mb/pg_wchar.h"