Drop extra separator from symlink install destination path

[cottsay]

The snippet for detecting the trailing / was taken from elsewhere in this file.

Before:

...
-- Install configuration: ""                  
-- Execute custom install script
-- Symlinking: /home/cottsay/rolling_ws/install/testpkg//libtestpkg.so.1.2.3
-- Symlinking: /home/cottsay/rolling_ws/install/testpkg//libtestpkg.a
-- Symlinking: /home/cottsay/rolling_ws/install/testpkg//testpkg
...

After:

...
-- Install configuration: ""
-- Execute custom install script
-- Symlinking: /home/cottsay/rolling_ws/install/testpkg/libtestpkg.so.1.2.3
-- Symlinking: /home/cottsay/rolling_ws/install/testpkg/libtestpkg.a
-- Symlinking: /home/cottsay/rolling_ws/install/testpkg/testpkg
...