Mention pkg-config in build requirements too

Fixes #740


git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5070 d4fa192b-c00b-0410-8231-f00ffab90ce4

doc/sphinx/installation/install.rst

@@ -91,6 +91,7 @@ installed. On a Debian or Ubuntu system these are:
 * xsltproc
 * groff-base
 * libpcre3-dev
+* pkg-config
 
 To install all these just type ``sudo apt-get install autotools-dev automake1.9 libtool autoconf libncurses-dev xsltproc groff-base libpcre3-dev``.