Fix charts on Windows

Local URI's should be prefixed with file://. Otherwise webkit and other
browsers as well (like firefox) will take the drive letter (C: - typical
for Windows) to be an unknown network protocol.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@22978 57a11ea4-9604-0410-9ed3-97b8803252fd