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/headers/tomcrypt_prng.h

@@ -193,6 +193,9 @@ unsigned long rng_get_bytes(unsigned char *out,
 
 int rng_make_prng(int bits, int wprng, prng_state *prng, void (*callback)(void));
 
+extern unsigned long (*ltc_rng)(unsigned char *out, unsigned long outlen,
+      void (*callback)(void));
+
 
 /* $Source$ */
 /* $Revision$ */