You can find that dll in: Qt_INSTALL_FOLDER\mingw\bin //you are going to need another dll from that folder "libgcc_s_dw2-1.dll"

And after that you need Qt's dll's from Qt_INSTALL_FOLDER\qt\bin (those from Qt_INSTALL_FOLDER\bin don't work with your application, those are used by Qt Creator)
Copy those dll's in the same folder as your .exe file
Note: You need to copy QtGuid4.dll for the debug build and QtGui4.dll for release build