[preservefiles] Mark this as an emergency module

- For C++ modules, don't need the module.desc file in the
   source repo, since the CMake macros can create it.

src/modules/preservefiles/CMakeLists.txt

@@ -8,4 +8,5 @@ calamares_add_plugin( preservefiles
     LINK_PRIVATE_LIBRARIES
         calamares
     SHARED_LIB
+    EMERGENCY
 )

src/modules/preservefiles/module.desc

@@ -1,5 +0,0 @@
----
-type:      "job"
-name:      "preservefiles"
-interface: "qtplugin"
-load:      "libcalamares_job_preservefiles.so"