Add FreeType libraries for Windows -system-freetype builds

Add a "CONFIG += qpa/basicunixfontdatase" to the -system-freetype block
so that the FreeType libraries are included in LIBS.

Task-number:  QTBUG-35767
Change-Id: I81e18e72e068d5ac240cba79bab880e50a033860
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>