Changed QLibrary::resolve() to return a function pointer.

Changed QLibrary::resolve() to return a function pointer.

According to the C++ standard, there is no guarantee that
you can cast between function pointers and void pointers
without data loss (section 5.2.10-6).

Change-Id: I27f4d835e4c8ca8ecca0d76cfea9ce34491956bd
Reviewed-on: http://codereview.qt.nokia.com/1995
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>