lib/crc32: prefix public functions

lib/crc32: prefix public functions

Make the publicly-visible crc32 library functions prefixed by ul_, such
as crc32() -> ul_crc32().
This is because it clashes with the crc32() function from zlib.
For newer versions of glib (2.50+) zlib and libblkid are required
dependencies and otherwise results in build failure when building
statically.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>