Ensure cooked_index_entry self-tests are run

While looking at code coverage for gdb, I noticed that the
cooked_index_entry self-tests were not run.  I tracked this down to a
formatting error in cooked-index-entry.c.

I suspect it might be better to use a macro to define these
initialization functions.  That would probably remove the possibility
for this kind of error.

gdb/dwarf2/cooked-index-entry.c

@@ -234,7 +234,8 @@ cooked_index_entry::write_scope (struct obstack *storage,
 }
 
 void _initialize_dwarf2_entry ();
-void _initialize_dwarf2_entry ()
+void
+_initialize_dwarf2_entry ()
 {
 #if GDB_SELF_TEST
   selftests::register_test ("cooked_index_entry::compare", test_compare);