Fix to include process.h on Windows.
Fix to include process.h on Windows.

I forgot a header on the Windows build to find _getpid().

Change-Id: I15dca3e798590402dcf3a4b8ae0a21b9a9350d45

blowekamp/SimpleITK,SimpleITK/SimpleITK,SimpleITK/SimpleITK,richardbeare/SimpleITK,blowekamp/SimpleITK,InsightSoftwareConsortium/SimpleITK,hendradarwin/SimpleITK,richardbeare/SimpleITK,SimpleITK/SimpleITK,richardbeare/SimpleITK,InsightSoftwareConsortium/SimpleITK,kaspermarstal/SimpleElastix,InsightSoftwareConsortium/SimpleITK,InsightSoftwareConsortium/SimpleITK,InsightSoftwareConsortium/SimpleITK,blowekamp/SimpleITK,InsightSoftwareConsortium/SimpleITK,hendradarwin/SimpleITK,SimpleITK/SimpleITK,InsightSoftwareConsortium/SimpleITK,InsightSoftwareConsortium/SimpleITK,richardbeare/SimpleITK,blowekamp/SimpleITK,hendradarwin/SimpleITK,hendradarwin/SimpleITK,richardbeare/SimpleITK,kaspermarstal/SimpleElastix,SimpleITK/SimpleITK,SimpleITK/SimpleITK,kaspermarstal/SimpleElastix,blowekamp/SimpleITK,kaspermarstal/SimpleElastix,richardbeare/SimpleITK,hendradarwin/SimpleITK,richardbeare/SimpleITK,richardbeare/SimpleITK,kaspermarstal/SimpleElastix,blowekamp/SimpleITK,hendradarwin/SimpleITK,SimpleITK/SimpleITK,SimpleITK/SimpleITK,kaspermarstal/SimpleElastix,hendradarwin/SimpleITK,hendradarwin/SimpleITK,blowekamp/SimpleITK,kaspermarstal/SimpleElastix,blowekamp/SimpleITK