Install pkg-config before attempting to build varnish
Install pkg-config before attempting to build varnish

Builds of the 4.1 Varnish branch were failing with the error
"configure.ac:123: error: pkg.m4 missing, please install pkg-config".

Change-Id: Iec93f0ae6405e6231027af60c45a02d0383afaa2

wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,stuartbman/mediawiki-vagrant,stuartbman/mediawiki-vagrant,stuartbman/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,stuartbman/mediawiki-vagrant,stuartbman/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,stuartbman/mediawiki-vagrant,stuartbman/mediawiki-vagrant