mpi4py: Suppress warnings in third-party code

Use vtk_third_party_warning_suppress to disable warnings when compiling
third-party C code in the mpi4py module.  Without this, there may be
many warnings with -Wunused-parameter.  To avoid modifying third-party
code, simply suppress them.

Change-Id: I216f8dae9e9001d22d5c5dd06aeb597e48baf836
Merge topic 'mpi4py-suppress-warnings' into master

644096bc mpi4py: Suppress warnings in third-party code