doc: add python3-filelock python3-pytest-xdist

Packages python3-filelock python3-pytest-xdist as required to run
'make tests'. Add them to the required packages list in the documentation.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/build/gcc.rst b/doc/build/gcc.rst
index a71f860..a0650a5 100644
--- a/doc/build/gcc.rst
+++ b/doc/build/gcc.rst
@@ -27,10 +27,11 @@
       device-tree-compiler dfu-util efitools flex gdisk graphviz imagemagick \
       liblz4-tool libgnutls28-dev libguestfs-tools libncurses-dev \
       libpython3-dev libsdl2-dev libssl-dev lz4 lzma lzma-alone openssl \
-      pkg-config python3 python3-asteval python3-coverage \
+      pkg-config python3 python3-asteval python3-coverage python3-filelock \
       python3-pkg-resources python3-pycryptodome python3-pyelftools \
-      python3-pytest python3-sphinxcontrib.apidoc python3-sphinx-rtd-theme \
-      python3-subunit python3-testtools python3-virtualenv swig uuid-dev
+      python3-pytest python3-pytest-xdist python3-sphinxcontrib.apidoc \
+      python3-sphinx-rtd-theme python3-subunit python3-testtools \
+      python3-virtualenv swig uuid-dev
 
 SUSE based
 ~~~~~~~~~~