seccomp: document the "filter_count" field

Add missing kernel-doc for the 'filter_count' field in struct seccomp.

seccomp.h:40: warning: Function parameter or member 'filter_count' not described in 'seccomp'

Fixes: c818c03b ("seccomp: Report number of loaded filters in /proc/$pid/status")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221202073953.14677-1-rdunlap@infradead.org