Turn addrandom()'s buf argument into unsigned char *

Data is usually represented as "unsigned char *" like genrandom().

dbrandom.c

@@ -141,7 +141,7 @@ out:
 	return ret;
 }
 
-void addrandom(char * buf, unsigned int len)
+void addrandom(unsigned char * buf, unsigned int len)
 {
 	hash_state hs;
 

dbrandom.h

@@ -29,7 +29,7 @@
 
 void seedrandom();
 void genrandom(unsigned char* buf, unsigned int len);
-void addrandom(char * buf, unsigned int len);
+void addrandom(unsigned char * buf, unsigned int len);
 void gen_random_mpint(mp_int *max, mp_int *rand);
 
 #endif /* DROPBEAR_RANDOM_H_ */