powerpc/sstep: Fix incorrect CONFIG symbol in scv handling

When I "fixed" the ppc64e build in Nick's recent patch, I typoed the
CONFIG symbol, resulting in one that doesn't exist. Fix it to use the
correct symbol.
Reported-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Fixes: 7fa95f9a ("powerpc/64s: system call support for scv/rfscv instructions")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200724131609.1640533-1-mpe@ellerman.id.au

arch/powerpc/lib/sstep.c

@@ -3382,7 +3382,7 @@ int emulate_step(struct pt_regs *regs, struct ppc_inst instr)
 		regs->msr = MSR_KERNEL;
 		return 1;
 
-#ifdef CONFIG_PPC64_BOOK3S
+#ifdef CONFIG_PPC_BOOK3S_64
 	case SYSCALL_VECTORED_0:	/* scv 0 */
 		regs->gpr[9] = regs->gpr[13];
 		regs->gpr[10] = MSR_KERNEL;