bionic-amd64-tf-m-build: Add networkx dependency

Add networkx python package to requirements_python3

Change-Id: I1daea0f56a85e5dcba8f485244ffcf77c8a39e50
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/bionic-amd64-tf-m-build/requirements_python3.txt b/bionic-amd64-tf-m-build/requirements_python3.txt
index b29388f..f53fdc3 100755
--- a/bionic-amd64-tf-m-build/requirements_python3.txt
+++ b/bionic-amd64-tf-m-build/requirements_python3.txt
@@ -17,3 +17,4 @@
 cysecuretools==4.1.0
 pyhsslms~=2.0.0
 kconfiglib==14.1.0
+networkx==2.5.1