eliminate MINGW pragma warning

http://build.opencv.org/builders/win_x86%5Brelease%5D--%5Bsse_%3D_ON%5D--%5Btbb_%3D_OFF%5D--%5Bgpu_%3D_OFF%5D--%5Bcompiler_%3D_mingw%5D--%5Btests_%3D_ON%5D-/builds/1236/