Genex: Fix processing multiple include directories for relative paths

Re-insert the semicolon which was removed during splitting.

Commit d777b8e7 (Genex: Allow relative paths in INSTALL_INTERFACE.,
2013-07-25) introduced the prefixItems method to allow relative paths
in the argument of the INSTALL_INTERFACE expression. That method was
buggy in that it did not re-introduce the semicolon separator in
the result.

This bug also affects paths which are already absolute in user code.
Merge topic 'fix-install-include-dirs-processing'

6f98f4a Genex: Fix processing multiple include directories for relative paths