powerpc/powermac: Mark variable x as unused

Since the value of x is never intended to be read, declare it with gcc
attribute as unused. Fix warning treated as error with W=1:

  arch/powerpc/platforms/powermac/bootx_init.c:471:21: error: variable ‘x’ set but not used [-Werror=unused-but-set-variable]
Suggested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powermac/bootx_init.c

@@ -468,7 +468,7 @@ void __init bootx_init(unsigned long r3, unsigned long r4)
 	boot_infos_t *bi = (boot_infos_t *) r4;
 	unsigned long hdr;
 	unsigned long space;
-	unsigned long ptr, x;
+	unsigned long ptr;
 	char *model;
 	unsigned long offset = reloc_offset();
 
@@ -562,6 +562,8 @@ void __init bootx_init(unsigned long r3, unsigned long r4)
 	 * MMU switched OFF, so this should not be useful anymore.
 	 */
 	if (bi->version < 4) {
+		unsigned long x __maybe_unused;
+
 		bootx_printf("Touching pages...\n");
 
 		/*