watter.c: Use size_t to silence a compiler warning

When compiled with -Wextra (or -Wsign-compare), we get a warning,
because strlen() returns size_t (usually unsigned), not int. Just use a
size_t to avoid that.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #6 from algernon/h/size-compare-fix

watter.c: Use size_t to silence a compiler warning