Added libgtk-3-dev to apt-get command

BUILDING.md

diff --git a/BUILDING.md b/BUILDING.md
index b93b465..6b2fb34 100644
--- a/BUILDING.md
+++ b/BUILDING.md
@@ -66,6 +66,7 @@
 	libfontconfig1-dev \
 	libx11-dev \
 	libxft-dev \
+	libgtk-3-dev \
 	perl \
 	liblist-moreutils-perl \
 	python \