Avoid warnings by defining prototypes for crc32 functions

crc32.c:14:6: warning: no previous prototype for function 'make_crc_table' [-Wmissing-prototypes]
void make_crc_table(void)
     ^
crc32.c:46:15: warning: no previous prototype for function 'update_crc' [-Wmissing-prototypes]
unsigned long update_crc(unsigned long crc,
              ^
crc32.c:61:15: warning: no previous prototype for function 'crc32' [-Wmissing-prototypes]
unsigned long crc32(unsigned char *buf, int len)
              ^
3 warnings generated.