powerpc/test_emulate_sstep: Fix build error

ppc64_book3e_allmodconfig fails with:

  arch/powerpc/lib/test_emulate_step.c: In function 'test_pld':
  arch/powerpc/lib/test_emulate_step.c:113:7: error: implicit declaration of function 'cpu_has_feature'
    113 |  if (!cpu_has_feature(CPU_FTR_ARCH_31)) {
        |       ^~~~~~~~~~~~~~~

Add an include of cpu_has_feature.h to fix it.

Fixes: b6b54b42 ("powerpc/sstep: Add tests for prefixed integer load/stores")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200724004109.1461709-1-mpe@ellerman.id.au

arch/powerpc/lib/test_emulate_step.c

@@ -8,6 +8,7 @@
 #define pr_fmt(fmt) "emulate_step_test: " fmt
 
 #include <linux/ptrace.h>
+#include <asm/cpu_has_feature.h>
 #include <asm/sstep.h>
 #include <asm/ppc-opcode.h>
 #include <asm/code-patching.h>