erofs-utils: fix "non-trivial designated initializers not supported"

erofs-utils: fix "non-trivial designated initializers not supported"

This partially reverts commit 79f6e168d94c ("erofs-utils: improve
compatibility and reduce header conflicts") since some C++ compiler
will complain:

include/erofs_fs.h: In function 'void erofs_check_ondisk_layout_definitions()':
include/erofs_fs.h:460:2: sorry, unimplemented: non-trivial designated initializers not supported

Let's just bypass this compile-time check for the C++ language since
only external programs may be written in C++.

Fixes: 79f6e168d94c ("erofs-utils: improve compatibility and reduce header conflicts")
Cc: Hongzhen Luo <hongzhen@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240627031343.3424030-1-hsiangkao@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>