mem_sec.c: portability fixup.

Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NBen Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/5493)

crypto/mem_sec.c

@@ -45,6 +45,9 @@
 #ifndef PAGE_SIZE
 # define PAGE_SIZE    4096
 #endif
+#if !defined(MAP_ANON) && defined(MAP_ANONYMOUS)
+# define MAP_ANON MAP_ANONYMOUS
+#endif
 
 #ifdef IMPLEMENTED
 static size_t secure_mem_used;