website: add pkg-config to Ubuntu dependencies

Corresponds to: https://gem5-review.googlesource.com/c/public/gem5/+/34777

Change-Id: I8396e4cc9210ff95e7f12e2aa57151bb3b213e49
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/34935
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
diff --git a/_pages/documentation/general_docs/building/index.md b/_pages/documentation/general_docs/building/index.md
index 1549c27..42d84fa 100644
--- a/_pages/documentation/general_docs/building/index.md
+++ b/_pages/documentation/general_docs/building/index.md
@@ -66,7 +66,7 @@
 ```
 sudo apt install build-essential git m4 scons zlib1g zlib1g-dev \
     libprotobuf-dev protobuf-compiler libprotoc-dev libgoogle-perftools-dev \
-    python-dev python-six python libboost-all-dev
+    python-dev python-six python libboost-all-dev pkg-config
 ```
 
 This will create an environment which uses Python 2.
@@ -79,7 +79,7 @@
 ```
 sudo apt install build-essential git m4 scons zlib1g zlib1g-dev \
     libprotobuf-dev protobuf-compiler libprotoc-dev libgoogle-perftools-dev \
-    python3-dev python3-six python-is-python3 libboost-all-dev
+    python3-dev python3-six python-is-python3 libboost-all-dev pkg-config
 ```
 
 This will create an environment which uses Python 3.