selftests/powerpc: ppc_asm.h: typo in the header guard

The guard macro __PPC_ASM_H in the header ppc_asm.h
doesn't match the #ifndef macro _PPC_ASM_H. The patch
makes them the same.
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/stringloops/asm/ppc_asm.h

 /* SPDX-License-Identifier: GPL-2.0 */
 #ifndef _PPC_ASM_H
-#define __PPC_ASM_H
+#define _PPC_ASM_H
 #include <ppc-asm.h>
 
 #ifndef r1