Add build dependency on tensorflow-sources to avoid build conflicts

By default, cmsBuild tries to build tensorflow-sources and
tensorflow-python3-sources in parallel, leading to conflicts with the
Bazel builds.
Declaring a build dependency on tensorflow-sources should force cmsBuild
to build these packages one after the other.

tensorflow-python3-sources.spec

@@ -12,6 +12,7 @@ Patch3: tensorflow-1.6.0-eigen-rename-sigmoid
 
 BuildRequires: bazel eigen protobuf gcc
 BuildRequires: py2-setuptools java-env
+BuildRequires: tensorflow-sources
 Requires: py2-numpy python3 py2-wheel
 
 %prep