add ltc_rng function pointer

the idea is to be able to easily provide a plug-in rng for a specific
platform without the need to touch the library.

src/misc/crypt/crypt_prng_rng_descriptor.c

@@ -0,0 +1,13 @@
+/* LibTomCrypt, modular cryptographic library -- Tom St Denis
+ *
+ * LibTomCrypt is a library that provides various cryptographic
+ * algorithms in a highly modular and flexible manner.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ *
+ * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
+ */
+#include "tomcrypt.h"
+
+unsigned long (*ltc_rng)(unsigned char *out, unsigned long outlen, void (*callback)(void));