Pass the correct argument to the observer in reread_symbols

Pass the correct argument to the observer in reread_symbols

This is actually a patch I found via another route.  Joel had asked me
to write a test, but I still have not found the time to do this.
Meanwhile, -Wunused-variable also found this error.

gdb/ChangeLog
2018-07-22  Tom Tromey  <tom@tromey.com>

	* symfile.c (reread_symbols): Notify iter, not objfile.