qt5base: unconditionally install Qt5printSupport if widgets are enabled

The qt5 build system currently unconditionally builds and installs into
staging Qt5PrintSupport if widgets are enabled, so ensure it also gets
installed into target to make sure we don't end up with dynamic linker
errors at runtime:

test: error while loading shared libraries: libQt5PrintSupport.so.5: cannot
open shared object file: No such file or directory

From src.pro:

!contains(QT_CONFIG, no-gui) {
        ..
        !wince*:!winrt {
            SUBDIRS += src_printsupport
            src_plugins.depends += src_printsupport
        }

}

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>