build/Makefile.msvcproj: Fix cleanup
build/Makefile.msvcproj: Fix cleanup

This removes the *.vs12.sourcefiles and *.vs12.sourcefiles.filters that
are generated during the process of generating the Visual Studio 2013
projects, so that 'make distcheck' won't complain about leftover files.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg