powerpc/test_emulate_step: Move extern declaration to sstep.h

fix checkpatch.pl warnings by moving extern declaration from source
file to headerfile.

Signed-off-by: Balamuruhan S <bala24@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200626095158.1031507-5-bala24@linux.ibm.com