install graphviz package to build SAI meta data (#25)

Author: lguohan

sonic-slave/Dockerfile

@@ -19,7 +19,7 @@ RUN apt-get update && apt-get install -y libreadline-dev texlive-latex-base texl
 RUN apt-get install -y cdbs
 
 # For SAI meta build
-RUN apt-get update && apt-get install -y libxml-simple-perl
+RUN apt-get update && apt-get install -y libxml-simple-perl graphviz
 
 # For linux build
 RUN apt-get update && apt-get install -y bc fakeroot build-essential devscripts quilt stgit