Update VMTKSlicerExtension extension

A space after -F seems to break the build:

http://slicer.cdash.org/viewBuildError.php?buildid=76281

Indeed, man ld says:

 -Fdir       Add dir to the list of directories in which to search for
             frameworks.  Directories specified with -F are searched in
             the order they appear on the command line and before the
             default search path. In Xcode4 and later, there can be a
             space between the -F and directory.
So we need to remove the space for backward compatibility.