[gtk-doc] common: add missing import

[Stefan Sauer <stefkost src gnome org>]

commit f8b1dad067ea1ea89d4bd92e00958ef3a61db3da
Author: Stefan Sauer <ensonic users sf net>
Date:   Wed Apr 12 07:44:38 2017 +0200

    common: add missing import

 gtkdoc/common.py |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/gtkdoc/common.py b/gtkdoc/common.py
index 36c5734..26b8d70 100644
--- a/gtkdoc/common.py
+++ b/gtkdoc/common.py
@@ -21,6 +21,7 @@
 
 import logging
 import os
+import subprocess
 
 from . import config