readthedocs: Install cmake to build the documentation

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/.readthedocs.yaml b/.readthedocs.yaml
index 96d651a..3cc3474 100644
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -13,6 +13,8 @@
 # Set the version of Python and other tools you might need
 build:
   os: ubuntu-20.04
+  apt_packages:
+    - cmake
   tools:
     python: "3.9"
   jobs: