Put the mem.h include back into crypto.h.

054e682675 removed the compatibility include of
mem.h in crypto.h. mem.h doesn't exist in upstream which defines these
functions in crypto.h instead. The compatibility include should probably be
restored to avoid causing all kinds of grief when porting consumers over.

Change-Id: Idfe0f9b43ebee5df22bebfe0ed6dc85ec98b4de0
Reviewed-on: https://boringssl-review.googlesource.com/4530
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/crypto.h

@@ -17,6 +17,10 @@
 
 #include <openssl/base.h>
 
+/* Upstream OpenSSL defines |OPENSSL_malloc|, etc., in crypto.h rather than
+ * mem.h. */
+#include <openssl/mem.h>
+
 
 #if defined(__cplusplus)
 extern "C" {