make freetype & fontconfig dependencies private again

exporting the freetype dependency caused qtwayland to break with
-qt-freetype, as the helper libraries' module pri files are not
installed (for good reasons)

after f9a80e06a, no actual user of FontDatabaseSupport needs access to
the transitive dependencies anyway (one of the headers has a fontconfig
dependency, but it's not used outside the module itself), so hiding them
again is just fine.

this partially reverts ec774500f.

Task-number: QTBUG-56666
Change-Id: I9e68a7e0725a92833b856c9ffdbec61c8aa5fed2
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

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