Add <stdalign.h> to c.h

This allows using the C11 constructs alignas and alignof (not done in
this patch).

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/46f05236-d4d4-4b4e-84d4-faa500f14691%40eisentraut.org

src/include/c.h

@@ -60,6 +60,7 @@
 
 /* System header files that should be available everywhere in Postgres */
 #include <inttypes.h>
+#include <stdalign.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>