Dedup include dir paths

Remove duplicates from include dir paths before setting
FlagExporterInfoProvider.  Duplicates often occur, for example due to
inheriting export_include_dirs from the static variant of a library to
the shared variant of the library that also gets the same
export_include_dirs.

Test: m checkbuild
Change-Id: I71f9e841547db64cbabcd95288ee93ef21fab4fb