Add missing prototypes for arc4random_buf and arc4random_uniform

Patch by Kevin McCarthy <signals@gentoo.org>

--- include/bsd/stdlib.h
+++ include/bsd/stdlib.h
@@ -45,6 +45,8 @@
 u_int32_t arc4random();
 void arc4random_stir();
 void arc4random_addrandom(u_char *dat, int datlen);
+void arc4random_buf(void *_buf, size_t n);
+u_int32_t arc4random_uniform(u_int32_t upper_bound);
 
 int dehumanize_number(const char *str, int64_t *size);