lz4: Fix compile warning comparison of distinct pointer types

lz4: Fix compile warning comparison of distinct pointer types

In file included from include/linux/bitops.h:22,
                 from include/log.h:15,
                 from include/linux/printk.h:4,
                 from include/common.h:20,
                 from lib/lz4_wrapper.c:6:
lib/lz4_wrapper.c: In function ‘ulz4fn’:
include/linux/kernel.h:184:17: warning: comparison of distinct pointer types lacks a cast
  (void) (&_min1 == &_min2);  \
                 ^~
lib/lz4_wrapper.c:104:18: note: in expansion of macro ‘min’
    size_t size = min((ptrdiff_t)block_size, end - out);
                  ^~~

Signed-off-by: Pali Rohár <pali@kernel.org>