windows build: Silence a false positive warning.

Tested by: jcorvel

* build/generator/gen_base.py
  (GeneratorBase.write_errno_table): Make this an instance method, rather than
    a static one.  Use self.errno_filter.
  (GeneratorBase.errno_filter): Provide default implementation.

* build/generator/gen_win.py
  (WinGeneratorBase.errno_filter): Override the default implementation.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1488803 13f79535-47bb-0310-9956-ffa450edef68