Classify known header file extensions as headers

Commit 328c0f65 (Simplify cmVisualStudio10TargetGenerator source
classification, 2012-03-19) introduced the first use of source
classification from cmGeneratorTarget (which originated as Makefile
generator logic) in a Visual Studio generator for handling of header
files.  Fix classification of header files to match known header
extensions instead of only the HEADER_FILE_ONLY property.  Make it
consistent with the "Header Files" source group.