bpf, x86: Fix "no previous prototype" warning

The arch_prepare_bpf_dispatcher function does not have a prototype, and
yields the following warning when W=1 is enabled for the kernel build.

  >> arch/x86/net/bpf_jit_comp.c:2188:5: warning: no previous \
  prototype for 'arch_prepare_bpf_dispatcher' [-Wmissing-prototypes]
        2188 | int arch_prepare_bpf_dispatcher(void *image, s64 *funcs, \
	int num_funcs)
             |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~

Remove the warning by adding a function declaration to include/linux/bpf.h.

Fixes: 75ccbef6 ("bpf: Introduce BPF dispatcher")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Björn Töpel <bjorn@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211117125708.769168-1-bjorn@kernel.org

include/linux/bpf.h

@@ -732,6 +732,7 @@ int bpf_trampoline_unlink_prog(struct bpf_prog *prog, struct bpf_trampoline *tr)
 struct bpf_trampoline *bpf_trampoline_get(u64 key,
 					  struct bpf_attach_target_info *tgt_info);
 void bpf_trampoline_put(struct bpf_trampoline *tr);
+int arch_prepare_bpf_dispatcher(void *image, s64 *funcs, int num_funcs);
 #define BPF_DISPATCHER_INIT(_name) {				\
 	.mutex = __MUTEX_INITIALIZER(_name.mutex),		\
 	.func = &_name##_func,					\