[vendor/x11]: Add build guards for other xlib files

vendor/x11/xlib/xlib_const.odin

@@ -1,4 +1,4 @@
-
+//+build linux, freebsd, openbsd
 package xlib
 
 // Special values for many types. Most of these constants

vendor/x11/xlib/xlib_keysym.odin

@@ -1,4 +1,4 @@
-
+//+build linux, freebsd, openbsd
 package xlib
 
 KeySym :: enum u32 {