Restore missing variable 'target_filename_ext' needed to determine if RPATH fix is necessary or not (#5824)

Signed-off-by: Steve Pham <82231385+spham-amzn@users.noreply.github.com>

cmake/Platform/Linux/runtime_dependencies_linux.cmake.in

@@ -10,6 +10,7 @@ cmake_policy(SET CMP0012 NEW) # new policy for the if that evaluates a boolean o
 
 function(ly_copy source_file target_directory)
     cmake_path(GET source_file FILENAME target_filename)
+    cmake_PATH(GET source_file EXTENSION target_filename_ext)
     cmake_path(APPEND target_file "${target_directory}" "${target_filename}")
     cmake_path(COMPARE "${source_file}" EQUAL "${target_file}" same_location)
     if(NOT ${same_location})