gl/cocoa: fix compiler warning
Use the reshape function after being defined. The other way
would have been to declare the reshape function in the header.
gstglwindow_cocoa.m: In function '-[GstGLNSView drawRect:]':
gstglwindow_cocoa.m:555: warning: 'GstGLNSView' may not respond to '-reshape'
gstglwindow_cocoa.m:555: warning: (Messages without a matching method signature
gstglwindow_cocoa.m:555: warning: will be assumed to return 'id' and accept
gstglwindow_cocoa.m:555: warning: '...' as arguments.)