Fix cmake config file to use @package_nspace@ (nonstd-lite issue 68)

nonstd-lite/nonstd-lite#68

Author: martinmoene

cmake/string-view-lite-config.cmake.in

@@ -2,6 +2,6 @@
 
 # Only include targets once:
 
-if( NOT TARGET @package_name@::@package_name@ )
+if( NOT TARGET @package_nspace@::@package_name@ )
     include( "${CMAKE_CURRENT_LIST_DIR}/@package_target@.cmake" )
 endif()