meson: and another missing file in ecore

they are so slowly caught by two reasons:

- .eo.c files are defining weak symbols, so no warnings when a symbol is
missing
- The APIs of the missing files are not tested, nor in examples in tree,
so not build and not discovered

src/lib/ecore/meson.build

@@ -181,6 +181,7 @@ ecore_src = [
   'efl_exe.c',
   'efl_thread.c',
   'efl_threadio.c',
+  'efl_appthread.c',
 ]
 
 if sys_windows == true