libblkid: fix gcc-7 warning -Wint-in-bool-context

libblkid: fix gcc-7 warning -Wint-in-bool-context

BLOCK_SIZE(sb) should be unsigned so that the left shift is defined.

This was the warning:

../libblkid/src/superblocks/exfat.c: In function 'probe_exfat':
../libblkid/src/superblocks/exfat.c:40:42: warning: '<<' in boolean context, did you mean '<' ? [-Wint-in-bool-context]
 #define CLUSTER_SIZE(sb) (BLOCK_SIZE(sb) << (sb)->bpc_bits)
                          ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
../libblkid/src/superblocks/exfat.c:122:14: note: in expansion of macro 'CLUSTER_SIZE'
  if (!sb || !CLUSTER_SIZE(sb))
              ^~~~~~~~~~~~

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>