Adding missing pkg-config package to Debian 9 build (#3517)

diff --git a/docker/compile/Dockerfile.debian9 b/docker/compile/Dockerfile.debian9
index fa2b2bc..f5e7b11 100644
--- a/docker/compile/Dockerfile.debian9
+++ b/docker/compile/Dockerfile.debian9
@@ -31,6 +31,7 @@
       libtool \
       libtool-bin \
       libcppunit-dev \
+      pkg-config \
       python-dev \
       python3-dev \
       software-properties-common \