Add missing header to ec/wnaf.c

a810d82 added calls to OPENSSL_malloc in this file, but openssl/mem.h
was missing.

Change-Id: I77e19e61e92b1e73702cb3eb93b9c6e22aca9596
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/41344
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/fipsmodule/ec/wnaf.c

@@ -72,6 +72,7 @@
 
 #include <openssl/bn.h>
 #include <openssl/err.h>
+#include <openssl/mem.h>
 #include <openssl/thread.h>
 
 #include "internal.h"