Prototype strcasestr in afsmonitor.c

Prototype strcasestr in afsmonitor.c

If we HAVE_STRCASESTR, we lack the prototype for strcasestr in
afsmonitor.c unless we def _GNU_SOURCE before including string.h on GNU
systems. To avoid side-effects or pulling in extra stuff, just prototype
strcasestr ourselves for the time being (if we have it) instead of
def'ing _GNU_SOURCE.

Reviewed-on: http://gerrit.openafs.org/595
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>