libext2fs: remove c99 idiom to fix build

libext2fs: remove c99 idiom to fix build

hashmap.c: In function ‘ext2fs_hashmap_free’:
hashmap.c:72:2: error: ‘for’ loop initial declarations are only allowed in C99 mode
  for (size_t i = 0; i < h->size; ++i) {
  ^
hashmap.c:72:2: note: use option -std=c99 or -std=gnu99 to compile your code
make[2]: *** [hashmap.o] Error 1

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>