Cleanup up non-prototyped functions (static) and const-correctness

More fixes based on the compiler directives -Wcast-qual -Wwrite-strings
-Wmissing-prototypes -Wmissing-declarations. Not everything with regards
to -Wcast-qual has been fixed as some have unwanted consequences for the
rest of the code.

library/gcm.c

@@ -1,7 +1,7 @@
 /*
  *  NIST SP800-38D compliant GCM implementation
  *
- *  Copyright (C) 2006-2012, Brainspark B.V.
+ *  Copyright (C) 2006-2013, Brainspark B.V.
  *
  *  This file is part of PolarSSL (http://www.polarssl.org)
  *  Lead Maintainer: Paul Bakker <polarssl_maintainer at polarssl.org>
@@ -123,7 +123,8 @@ static const uint64_t last4[16] =
     0x9180, 0x8da0, 0xa9c0, 0xb5e0
 };
 
-void gcm_mult( gcm_context *ctx, const unsigned char x[16], unsigned char output[16] )
+static void gcm_mult( gcm_context *ctx, const unsigned char x[16],
+                      unsigned char output[16] )
 {
     int i = 0;
     unsigned char z[16];