Fix PARAVIEW_EXTERNAL_PLUGIN_DIRS CMake Macro

Allow PARAVIEW_EXTERNAL_PLUGIN_DIRS to be the Plugin directory
and not only its parent. It use to work fine on Unix system
as the double / was properly ingored but on Windows it will
simply failed.

Change-Id: Ia14830b3e2ed7f253e6b1a01977be4017b85a443
Merge topic 'fix_cmake_external_plugin_macro' into next

b817d6c Fix PARAVIEW_EXTERNAL_PLUGIN_DIRS CMake Macro