[Development] qtwebglplugin fails to build in Coin on Windows (5.13.2 branch)

[Konstantin Tokarev]

Following error had reproduced several times for me:

qwebglintegration_p.h:43:10: fatal error: QtFontDatabaseSupport/private/qwindowsfontdatabase_p.h: No such file or directory

Both MinGW and MSVC seem to be affected.
Example build logs:

https://testresults.qt.io/logs/qt/qtwebglplugin/87539f0467ac06174008e7eacaf2ba824b68351e/WindowsWindows_7x86WindowsWindows_7x86Mingw73qtci-windows-7-x86-3-b142bdDisableTests/58cdeff5cfc2360d3d8f7d496a8c2f08d3c92441/build_1571837037/log.txt.gz

https://testresults.qt.io/logs/qt/qtwebglplugin/87539f0467ac06174008e7eacaf2ba824b68351e/WindowsWindows_10x86_64WindowsWindows_10x86_64MSVC2015qtci-windows-10-x86_64-14-61643fDisableTests/e8e1bb3c60e19e1ce500d948d1bcf0d385710421/build_1571867849/log.txt.gz

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Regards,
Konstantin