Fix find_and_open_source link error

Hi,

A recent change in gdb has rendered a function invisible to
insight, so 
I've found a suitable replacement and committed the attached
patch.

Keith

ChangeLog
2008-04-21  Keith Seitz  <keithsredhat.com>

         * generic/gdbtk-cmds.c (symtab_to_filename): Call
gdb's
         open_source_file instead of find_and_open_source.