VirtualBox: requires procps package on Windows/Cygwin

Add procps to the list of Cygwin packages required
to run the VirtualBox version of MOS on Windows systems.

This fix is being applied to the 6.0 docs but it would
be nice to cherrypick it to 5.1.1 as well.

Change-Id: I1e484958f0f03ac3928dca095f16b4338b579bf7
Closes-Bug:1380687
Merge "VirtualBox: requires procps package on Windows/Cygwin"