f2fs: avoid unused f2fs_show_compress_options()

LKP reports:

   fs/f2fs/super.c:1516:20: warning: unused function 'f2fs_show_compress_options' [-Wunused-function]
   static inline void f2fs_show_compress_options(struct seq_file *seq,

Fix this issue by covering f2fs_show_compress_options() with
CONFIG_F2FS_FS_COMPRESSION macro.

Fixes: 4c8ff709

 ("f2fs: support data compression")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>