Fix Qt5 libs linking to Qt5WebEngineCore.dll for Windows

MSVC linker needs file extension, otherwise it searches for .obj
files, but Qt has Qt5*.lib files.

Change-Id: I4fd5e1d6001ea309da456c43ffc79cb5944a1ef3
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>