softlockup: move 'one' to the softlockup section in sysctl.c

CONFIG_SOFTLOCKUP=y || CONFIG_DETECT_HUNG_TASKS=y is now the only user
of the 'one' constant in kernel/sysctl.c. Move it to the softlockup
block of constants.

This fixes a GCC warning.

Signed-off-by: Ingo Molnar <mingo@elte.hu>