[linux] clean up unused includes (leanflutter#260)

* [linux] clean up unused includes

- cairo is unused since leanflutter#250
- utsname and cstring are leftovers from the flutter plugin template

* [linux] change the last remaining strcmp() to g_strcmp0()

While one PR was changing strcmp()'s to g_strcmp0(), this piece of code
was added in another PR. Use the same method here as well, just for the
sake of aesthetics and consistency.

linux/window_manager_plugin.cc

@@ -2,10 +2,6 @@
 
 #include <flutter_linux/flutter_linux.h>
 #include <gtk/gtk.h>
-#include <sys/utsname.h>
-
-#include <cairo/cairo.h>
-#include <cstring>
 
 #define WINDOW_MANAGER_PLUGIN(obj)                                     \
   (G_TYPE_CHECK_INSTANCE_CAST((obj), window_manager_plugin_get_type(), \
@@ -847,7 +843,7 @@ static void window_manager_plugin_handle_method_call(
     response = grab_keyboard(self);
   } else if (g_strcmp0(method, "ungrabKeyboard") == 0) {
     response = ungrab_keyboard(self);
-  } else if (strcmp(method, "setBrightness") == 0) {
+  } else if (g_strcmp0(method, "setBrightness") == 0) {
     response = set_brightness(self, args);
   } else {
     response = FL_METHOD_RESPONSE(fl_method_not_implemented_response_new());