vcl headers via triangular brackets instead of quotes

Age	Commit message (Expand)	Author
2017-09-15	consistent naming of externals: ConvertTextToNumber->ct2n	Michael Stahl
2015-06-17	Fix location of bundled ct2n extension	Stephan Bergmann
Chris Sherlock
2015-11-10	loplugin:nullptr (automatic rewrite; Mac-specific code)	Stephan Bergmann
2015-09-21	convert Link<> to typed	Noel Grandin
2015-08-13	Remove #ifdef ENABLE_TRACING code	Stephan Bergmann