base: Remove the curTick prototype from base/statistics.hh.

base: Remove the curTick prototype from base/statistics.hh.

This prototype might convince the compiler that it should refer to
curTick indirectly through the linker, but curTick is inline (and making
it not has very high overhead), so there's a decent chance no non-inline
version will be emitted.

Change-Id: Iab5aacb145d4a974bc1bc0abdf7275c40fbb9c38
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38997
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>