crcsync: fix constness warning.

/home/rusty/devel/cvs/ccan/ccan/crcsync/test/run-crash.c:Compile gave warnings:
/home/rusty/devel/cvs/ccan/ccan/crcsync/test/run-crash.c: In function ‘main’:
/home/rusty/devel/cvs/ccan/ccan/crcsync/test/run-crash.c:30:3: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
/home/rusty/devel/cvs/ccan/ccan/crcsync/test/run-crash.c:35:3: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>