powerpc/ftrace: Don't include ftrace.o for CONFIG_FTRACE_SYSCALLS

Since commit 7bea7ac0 ("powerpc/syscalls: Fix syscall tracing")
ftrace.o is not needed anymore for CONFIG_FTRACE_SYSCALLS.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/275932a5d61543b825ff9a64f61abed6da5d4a2a.1652074503.git.christophe.leroy@csgroup.eu