locking/seqlock: Tweak DEFINE_SEQLOCK() kernel doc

ctags creates a warning:
|ctags: Warning: include/linux/seqlock.h:738: null expansion of name pattern "\2"

The DEFINE_SEQLOCK() macro is passed to ctags and being told to expect
an argument.

Add a dummy argument to keep ctags quiet.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lkml.kernel.org/r/20200924154851.skmswuyj322yuz4g@linutronix.de