Fix build on macos

I'm not sure entirely sure why arc4random_buf needs to be forward
declared, but these changes are needed for macos builds to work.

Fixed build errors:

include/random.h:4:1: error: unknown type name 'ssize_t'; did you mean 'size_t'?
ssize_t get_random_bytes(unsigned char *buf, size_t nbytes);
^~~~~~~

src/random-arc4.c:7:2: error: implicit declaration of function 'arc4random_buf'
is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        arc4random_buf(buf, nbytes);

Signed-off-by: William Casarin <jb55@jb55.com>

include/random.h

@@ -1,6 +1,8 @@
 #ifndef RANDOM_H
 #define RANDOM_H
 
+#include <sys/types.h>
+
 ssize_t get_random_bytes(unsigned char *buf, size_t nbytes);
 
 #endif

src/random-arc4.c

@@ -1,6 +1,8 @@
 #include <stdlib.h>
 #include "random.h"
 
+void arc4random_buf(void*, size_t);
+
 ssize_t
 get_random_bytes(unsigned char *buf, size_t nbytes)
 {