The macro `PATH_MAX` macro is typically defined in the <limits.h>
header by the system's libc implementation. While we do not
include it right now, glibc indirectly includes it via other
headers already and thus compilation of the file succeeds. For
other libc implementations this may not be the case, which would
then lead to a compilation error. This is the case for musl libc.

Explicitly include <limits.h> to fix this.

Signed-off-by: Patrick Steinhardt <ps@pks.im>