freebsd: add std.c.arc4random_buf

Author: mikdusan

lib/std/c/freebsd.zig

@@ -15,6 +15,7 @@ pub extern "c" fn pthread_getthreadid_np() c_int;
 pub extern "c" fn pthread_set_name_np(thread: std.c.pthread_t, name: [*:0]const u8) void;
 pub extern "c" fn pthread_get_name_np(thread: std.c.pthread_t, name: [*:0]u8, len: usize) void;
 pub extern "c" fn pipe2(fds: *[2]fd_t, flags: u32) c_int;
+pub extern "c" fn arc4random_buf(buf: [*]u8, len: usize) void;
 
 pub extern "c" fn posix_memalign(memptr: *?*anyopaque, alignment: usize, size: usize) c_int;
 pub extern "c" fn malloc_usable_size(?*const anyopaque) usize;