bpf: Correctly annotate implicit fall through in bpf_base_func_proto

There is a plan to build the kernel with -Wimplicit-fallthrough and
this place in the code produced a warnings (W=1).

To preserve as much of the existing comment only change a ‘:’ into a ‘,’.
This is enough change, to match the regular expression expected by GCC.

This commit removes the following warning:

  net/core/filter.c:5310:6: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>