coresight: Fix undeclared variable warnings from sparse checker

coresight: Fix undeclared variable warnings from sparse checker

Including the header with the declarations fixes the following warning
with a C=1 build:

  coresight-cfg-afdo.c:102:27: warning: symbol 'strobe_etm4x' was not declared. Should it be static?
  coresight-cfg-afdo.c:141:26: warning: symbol 'afdo_etm4x' was not declared. Should it be static?

Signed-off-by: James Clark <james.clark@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20231123120459.287578-4-james.clark@arm.com