Fix compilation with pycairo 1.17.x

--- a/pitivi/coptimizations/renderer.c    2017-04-29 11:33:23.000000000 +0200
+++ b/pitivi/coptimizations/renderer.c   2019-03-09 14:34:05.868654440 +0100
@@ -4,7 +4,6 @@
 #include <py3cairo.h>
 #include <gst/gst.h>

-static Pycairo_CAPI_t *Pycairo_CAPI;
 static GObjectClass * gobject_class;

 /*