selftests/powerpc: Remove repeated word in comments

Remove the repeated word "not" in comments.
Signed-off-by: Shaomin Deng <dengshaomin@cdjrlc.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221029094643.5595-1-dengshaomin@cdjrlc.com

tools/testing/selftests/powerpc/include/pkeys.h

@@ -24,7 +24,7 @@
 #undef PKEY_DISABLE_EXECUTE
 #define PKEY_DISABLE_EXECUTE	0x4
 
-/* Older versions of libc do not not define this */
+/* Older versions of libc do not define this */
 #ifndef SEGV_PKUERR
 #define SEGV_PKUERR	4
 #endif