gtk: use the standard path for including Adwaita header file

As pointed out by @tristan957 the standard path for including the
Adwaita header file is simply "adwaita.h". While it may have been
necessary in the past to use a non-standard include path, that no longer
appears to be the case.

src/apprt/gtk/c.zig

@@ -4,7 +4,7 @@ const build_options = @import("build_options");
 pub const c = @cImport({
     @cInclude("gtk/gtk.h");
     if (build_options.adwaita) {
-        @cInclude("libadwaita-1/adwaita.h");
+        @cInclude("adwaita.h");
     }
 
     if (build_options.x11) {