QPlatformThemePlugin's symbols are not Qt_5_PRIVATE_API versioned

As QPlatformThemePlugin is still private API they symbols under (at least) Linux should be versioned with Qt_5_PRIVATE_API. Example: see https://anonscm.debian.org/cgit/pkg-kde/qt/qtbase.git/tree/debian/libqt5gui5.symbols#n2807 lines 2807 to 2814.

At least from my point of view there are two possible solutions for this:

1) Probably the easiest one: add the necessary flags in the relevant .pro files to get symbols properly marked. I really don't know/remember how to do this, but thiago will surely know as he did the code that gets symbols versioned.

2) Possibly more complicated, but that will make downstreams' lives easier (distros and desktop environments): make the API public. Now every desktop environment wanting to provide a platform theme has to depend on private APIs which are not considered stable (although they have probably been for quite long, at least for this part of the code). Distros will not need to recompile each of this themes every time a minor release happens.