doc: included libgtk-3-dev in the list of dependencies
diff --git a/BUILDING.md b/BUILDING.md
index b1a7ecc..a2c9cb7 100644
--- a/BUILDING.md
+++ b/BUILDING.md
@@ -23,6 +23,7 @@
  * FreeType
  * Xft (libXft + libX11)
  * fontconfig
+ * libgtk-3-dev
 
 For the [regression testing and benchmarking](README.developers.md#running-tests) you will need:
  * Perl + List::MoreUtils