base: Remove unnecessary includes from base/loader/symtab.[cc|hh].

base: Remove unnecessary includes from base/loader/symtab.[cc|hh].

These were either completely unnecessary, or headers in the .cc which
had already been included in the .hh and were not needed beyond their
use in the .hh.

Change-Id: Ic95e29f3fdd8cab00ab93d254d2e1c25aacf4632
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40619
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>