Only import tkinter when needed

* Otherwise this could fail if tkinter isn't available on platforms where we're
  not even going to run the test

util/test/tests/D3D12/D3D12_RGP_Capture.py

@@ -1,7 +1,6 @@
 import renderdoc as rd
 import rdtest
 import os
-import tkinter
 
 
 class D3D12_RGP_Capture(rdtest.TestCase):
@@ -19,6 +18,8 @@ class D3D12_RGP_Capture(rdtest.TestCase):
             rdtest.log.print("RGP capture not tested")
             return
 
+        import tkinter
+
         # On D3D12 we need to create a real window
         window = tkinter.Tk()
         window.geometry("1280x720")