demandimport: blacklist gtk

Demandimport breaks gtk.  You get a meaningless error about
'failed loading gobject\_gobject.pyd'.  Mercurial does not use gtk,
but this trips up many extension writers.

mercurial/demandimport.py

@@ -127,6 +127,8 ignore = [
     # imported by profile, itself imported by hotshot.stats,
     # not available under Windows
     'resource',
+    # this trips up many extension authors
+    'gtk',
     ]
 
 def enable():