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

include/linux/seqlock.h

@@ -821,7 +821,7 @@ typedef struct {
 	} while (0)
 
 /**
- * DEFINE_SEQLOCK() - Define a statically allocated seqlock_t
+ * DEFINE_SEQLOCK(sl) - Define a statically allocated seqlock_t
  * @sl: Name of the seqlock_t instance
  */
 #define DEFINE_SEQLOCK(sl) \