sched: might_sleep(): Make file parameter const char *

Fixes a warning when building with g++:

 warning: deprecated conversion from string constant to 'char*'

And the file parameter use is constant, so mark it as such.

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
Cc: peterz@infradead.org
LKML-Reference: <20091223110818.442d848e@marrow.netinsight.se>
Signed-off-by: Ingo Molnar <mingo@elte.hu>