mem-cache: Removed default arg from get() in prefetch/base.hh

mem-cache: Removed default arg from get() in prefetch/base.hh

commit b0d1643 caused building against NULL to break due to
NULLIsa::GuestByteOrder not being defined.

Removal of default argument in src/mem/cache/prefetch/base.hh fixes
this.

Change-Id: I99a4abb4be1418fadec145481164f7caa3334ca0
Signed-off-by: Ryan Gambord
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17070
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>