nbd/003: fix compiling error with gcc version 4.8.5

nbd/003: fix compiling error with gcc version 4.8.5

cc  -O2 -Wall -Wshadow   -o mount_clear_sock mount_clear_sock.c
mount_clear_sock.c: In function ‘main’:
mount_clear_sock.c:39:2: error: ‘for’ loop initial declarations are only allowed in C99 mode
  for (int i = 0; i < loops; i++) {
  ^
mount_clear_sock.c:39:2: note: use option -std=c99 or -std=gnu99 to compile your code

Signed-off-by: Yi Zhang <yi.zhang@redhat.com>