Unconditionally define _initialize_addrmap

The way that init.c is generated does not allow for an initialization
function to be conditionally defined -- doing so will result in a link
error.

This patch fixes a build problem that arises from such a conditional
definition.  It can be reproduce with --disable-unit-tests.

gdb/addrmap.c

@@ -681,11 +681,13 @@ test_addrmap ()
 }
 
 } // namespace selftests
+#endif /* GDB_SELF_TEST */
 
 void _initialize_addrmap ();
 void
 _initialize_addrmap ()
 {
+#if GDB_SELF_TEST
   selftests::register_test ("addrmap", selftests::test_addrmap);
-}
 #endif /* GDB_SELF_TEST */
+}