Make matplotlib targets depend on gtk

Otherwise matplotlib complains it has no graphical backend and won't pop
up a plot.

Change-Id: I73b024c957ea87d1f440f1b73f0806440655c0fd
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/y2020/control_loops/python/BUILD b/y2020/control_loops/python/BUILD
index 8a1f737..3833536 100644
--- a/y2020/control_loops/python/BUILD
+++ b/y2020/control_loops/python/BUILD
@@ -122,6 +122,7 @@
     deps = [
         "//frc971/control_loops/python:controls",
         "@pip//matplotlib",
+        "@pip//pygobject",
     ],
 )