Add libc6-dev to bindep

The python-builder base image was updated to no longer install
recommends. This is inline with the other Infra images and keeps
image sizes smaller. gcc recommended libc6-dev - but it turns out
we need that for limits.h for one of our depends. Add it to fix
our image builds.

Change-Id: I97950d71bc455c269490812c6597fbe432641733

Author: emonty

bindep.txt

@@ -2,6 +2,7 @@
 # see https://docs.openstack.org/infra/bindep/ for additional information.
 
 gcc [compile test]
+libc6-dev [compile test platform:dpkg]
 libffi-devel [platform:rpm]
 libffi-dev [compile test platform:dpkg]
 libffi6 [platform:dpkg]