Fix deployment of qtwebengine_locales with make install

The install_locales target silently fails when running make install
with the message "install: omitting directory."
Add all the translations to the files variable and set the path to
$$[QT_INSTALL_TRANSLATIONS]/qtwebengine_locales.

Change-Id: I375077982d9b3e2154cf90ae13142bb42a07e38d
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>