std.c: add IPPROTO_RAW for Darwin platforms

IPPROTO_RAW (255) was missing from the Darwin/macOS IPPROTO struct,
even though it is defined in system headers and supported by the platform.
This is a commonly used protocol for raw IP sockets.

lib/std/c.zig

@@ -6026,6 +6026,7 @@ pub const IPPROTO = switch (native_os) {
         pub const UDP = 17;
         pub const IP = 0;
         pub const IPV6 = 41;
+        pub const RAW = 255;
     },
     .freebsd => struct {
         /// dummy for IP