Add back some library flags lost in r410751.

Some .pri files still get some of their information from Qt4's configure
script by default. Since we are not using it for the sqldriver ports, both
qt4-odbc-plugin and qt4-sqlite3-plugin were no longer linking against
libodbc.so and libsqlite3.so.

Fix it by explicitly passing those via LDFLAGS.

Thanks to Luca Pizzamiglio <luca.pizzamiglio@gmail.com> for the report.