MNG image plugin will not build when defining MNG_BUILD_DLL

When compiling Qt with VisualC++ 2010 the MNG image plugin will not compile
using an external libMNG (.\configure.exe -system-mng) that was compiled using
the macro MNG_BUILD_DLL (as is suggested on that plattform).

Putting the macro MNG_DECL before every definition of the callback in
qmnghandler.cpp fixes the issue (see the attached file).