use I3__FILE__ for DLOG, leave __FILE__ as is

See also commit 0e752070ac, which broke
source code listings in gdb unless you cd into i3/src. This should give
us best of both :-).

src/render.c

@@ -1,4 +1,5 @@
-#line 2 "render.c"
+#undef I3__FILE__
+#define I3__FILE__ "render.c"
 /*
  * vim:ts=4:sw=4:expandtab
  *