libc: add header for ffsl and ffsll

We have an implementation of those functions that external users can call at
will.  But because they are not anywhere in our headers, OSv users will not
find the correct function signature. Man page says:

       #include <string.h>

       int ffsl(long int i);

       int ffsll(long long int i);

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>