make the dll_copy generated file a PRIVATE target_source

This file is creating an error when trying to install as explained here https://stackoverflow.com/questions/60736689/cmake-target-sources-and-install.

build-utils/cmake/shared_library.cmake

@@ -43,6 +43,6 @@ function(target_copy_shared_libraries _TARGET)
       VERBATIM)
 
       # This allows to create a dependency with the command above, so command is executed again when target is built AND a DLL changed
-    target_sources(${_TARGET} PUBLIC "${CMAKE_CURRENT_BINARY_DIR}/${_TARGET}_dll_copy")
+    target_sources(${_TARGET} PRIVATE "${CMAKE_CURRENT_BINARY_DIR}/${_TARGET}_dll_copy")
   endif()
-endfunction()
+endfunction()