Install pkg-config in Debian/Ubuntu containers

It is needed to proper detect the grcc full path
diff --git a/tests/dockerfiles/Ubuntu_16_04.docker b/tests/dockerfiles/Ubuntu_16_04.docker
index e56b87f..28e01a6 100644
--- a/tests/dockerfiles/Ubuntu_16_04.docker
+++ b/tests/dockerfiles/Ubuntu_16_04.docker
@@ -5,6 +5,7 @@
     cmake \
     autoconf \
     libtool \
+    pkg-config \
     build-essential \
     python-docutils \
     libcppunit-dev \