package/qt5/qt5webengine: fix translations target install path

package/qt5/qt5webengine: fix translations target install path

Through the evolution of the qt5webengine package patch ([1], [2])
until the initial commit ([3]) the translations target install
path got mangled resulting in a double trailing qtwebengine_locales
path:
  /usr/translations/qtwebengine_locales/qtwebengine_locales
Instead of:
  /usr/translations/qtwebengine_locales

Fixes the translations runtime access failure resulting in the
following warning:

  WARNING:resource_bundle_qt.cpp(116): locale_file_path.empty() for locale

[1] http://lists.busybox.net/pipermail/buildroot/2015-July/132010.html
[2] https://patchwork.ozlabs.org/patch/640633
[3] https://git.buildroot.net/buildroot/commit/?id=89080bac9bc47946a09c1e74f2f872363bf6785b

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>