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>

src/plugins/platforms/windows/windows.pri

@@ -124,6 +124,7 @@ contains(QT_CONFIG, freetype) {
     SOURCES += \
                $$PWD/qwindowsfontdatabase_ft.cpp
 } else:contains(QT_CONFIG, system-freetype) {
+    CONFIG += qpa/basicunixfontdatabase
     include($$QT_SOURCE_TREE/src/platformsupport/fontdatabases/basic/basic.pri)
     HEADERS += \
                $$PWD/qwindowsfontdatabase_ft.h