proc: enclose desc variable of show_stat() in CONFIG_SPARSE_IRQ

Impact: restructure code to fix compiler warning

commit 240d367b moved desc usage point
into #ifdef CONFIG_SPARSE_IRQ.

Eliminate the desc variable, otherwise following warning happens:

 fs/proc/stat.c: In function 'show_stat':
 fs/proc/stat.c:31: warning: unused variable 'desc'

[ akpm: cleaned up the patch to remove #ifdef ]

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>