fsck.gfs2: Fix 'initializer element is not constant' build error
    
    This error occurs when gfs2-utils is compiled with -std options more
    recent than gnu89:
    
        CC       fsck_gfs2-main.o
      main.c:39:38: error: initializer element is not constant
       struct osi_root dup_blocks = (struct osi_root) { NULL, };
                                            ^
      main.c:40:35: error: initializer element is not constant
       struct osi_root dirtree = (struct osi_root) { NULL, };
                                         ^
      main.c:41:37: error: initializer element is not constant
       struct osi_root inodetree = (struct osi_root) { NULL, };
                                         ^
    As far as I can tell, with C89/gnu89 the use of a cast in this context
    is undefined behaviour and the later standards are more strict about it,
    hence the error. As the standards specify that members of objects with
    static storage duration are zeroed/NULLed anyway, the initializers can
    be removed to achieve the intended result.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>