seccomp: use a static inline for a function stub

Fixes this error message when CONFIG_SECCOMP is not set:

arch/powerpc/kernel/ptrace.c: In function 'do_syscall_trace_enter':
arch/powerpc/kernel/ptrace.c:1713:2: error: statement with no effect [-Werror=unused-value]

Change-Id: I538d010f83b0632028bc9df02437ed5149130e92
Signed-off-by: Stephen Rothwell <sfr@ozlabs.au.ibm.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>