Minor cleanup of symtab code/includes.

Minor cleanup of symtab code/includes.

base/loader/symtab.cc:
base/loader/symtab.hh:
    Get rid of old unused calls.
cpu/simple_cpu/simple_cpu.hh:
    No need to include base/loader/symtab.hh
kern/linux/linux_system.cc:
kern/tru64/tru64_system.cc:
    Include base/loader/symtab.hh (since it's no longer included in system.hh)
sim/system.hh:
    Replace include of base/loader/symtab.hh with forward class decl.

--HG--
extra : convert_revision : 3a778c2f409ec94e3b00eaa9b3859943cb39918c