qt5base: install libQt5XcbQpa.so for xcb backend

When the XCB backend is selected, the libqxcb.so plugin is installed,
and is linked against libQt5XcbQpa.so. However, until, Buildroot was
not installing this library. This commit fixes this.

[Thomas: tweak commit log.]

Signed-off-by: Matthew Shyu <matthew.shyu@amlogic.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>