src/t_enospc.c: Fix an error for the loop initialization declaration

src/t_enospc.c: Fix an error for the loop initialization declaration

When I compiled xfstests using the gcc(version 4.8.5), the following error occurred:
t_enospc.c: In function 'enospc_test':
t_enospc.c:88:2: error: 'for' loop initial declarations are only allowed in C99 mode
  for (int i = 0; i < size; i++) {
  ^

Signed-off-by: Feiyu Zhu <zhufy.jy@cn.fujitsu.com>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>