bpf: Fix sparse warning for bpf_kptr_xchg_proto

Kernel Test Robot complained about missing static storage class
annotation for bpf_kptr_xchg_proto variable.

sparse: symbol 'bpf_kptr_xchg_proto' was not declared. Should it be static?

This caused by missing extern definition in the header. Add it to
suppress the sparse warning.

Fixes: c0a5a21c

 ("bpf: Allow storing referenced kptr in map")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220511194654.765705-2-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>