Provide a stub random_byte() to make 'testbn' compile again.

The function bignum_random_in_range() is new to sshbn.c since I last
tried to run the bignum test code.

sshbn.c

@@ -1892,6 +1892,12 @@ void modalfatalbox(const char *p, ...)
     exit(1);
 }
 
+int random_byte(void)
+{
+    modalfatalbox("random_byte called in testbn");
+    return 0;
+}
+
 #define fromxdigit(c) ( (c)>'9' ? ((c)&0xDF) - 'A' + 10 : (c) - '0' )
 
 int main(int argc, char **argv)