bpf: Hide unused bpf_patch_call_args

This function is only used when CONFIG_BPF_JIT_ALWAYS_ON is disabled, but
CONFIG_BPF_SYSCALL is enabled. When both are turned off, the prototype is
missing but the unused function is still compiled, as seen from this W=1
warning:

  [...]
  kernel/bpf/core.c:2075:6: error: no previous prototype for 'bpf_patch_call_args' [-Werror=missing-prototypes]
  [...]

Add a matching #ifdef for the definition to leave it out.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20230602135128.1498362-1-arnd@kernel.org