doc: add pkg-config to the build dependencies

tools/Makefile uses pkg-config.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/build/gcc.rst b/doc/build/gcc.rst
index e03e0b9..0cdc307 100644
--- a/doc/build/gcc.rst
+++ b/doc/build/gcc.rst
@@ -26,8 +26,8 @@
     sudo apt-get install bc bison build-essential coccinelle \
       device-tree-compiler dfu-util efitools flex gdisk graphviz imagemagick \
       liblz4-tool libguestfs-tools libncurses-dev libpython3-dev libsdl2-dev \
-      libssl-dev lz4 lzma lzma-alone openssl python3 python3-coverage \
-      python3-pycryptodome python3-pyelftools python3-pytest \
+      libssl-dev lz4 lzma lzma-alone openssl pkg-config python3 \
+      python3-coverage python3-pycryptodome python3-pyelftools python3-pytest \
       python3-sphinxcontrib.apidoc python3-sphinx-rtd-theme python3-virtualenv \
       swig