Fix creation of symbolic links in rpm scripts.

Fixes trac issue [#10784](http://trac.mantidproject.org/mantid/ticket/10784)

Tester: The best check here is to look at the contents of the bin directory of the installed package. Both `MantidPlot` and `mantidplot` should be symbolic links to the `launch_mantidplot.sh` script. This script then deals with the TCMALLOC wrapper around the actually executable `MantidPlot_exe`