Since commit 7e92e01b7245 ("powerpc: Provide syscall wrapper")
landed in v6.1, syscall wrapper is enabled on powerpc. Commit
94746890202c ("powerpc: Don't add __powerpc_ prefix to syscall
entry points") , that drops the prefix to syscall entry points,
also landed in the same release. So, add the missing empty
SYS_PREFIX prefix definition for powerpc, to fix some fentry
and kprobe selftests.
Signed-off-by: Saket Kumar Bhaskar <skb99@xxxxxxxxxxxxx>