perf/bpf: Change the !CONFIG_BPF_SYSCALL stubs to static inlines

Otherwise the compiler will be unhappy if they go unused,
which they do on allnoconfigs.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Kyle Huey <me@kylehuey.com>
Link: https://lore.kernel.org/r/ZhkE9F4dyfR2dH2D@gmail.com