lib: fix compilation when strings.h is included

In a systems that provide strings.h and it is included
together with sbi_bitops.h the compilation error appears.
The ffs() and fls() are provided by strings.h

Signed-off-by: Petro Karashchenko <petro.karashchenko@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>