Fix various signed/unsigned gcc warnings

Some of these could affect filesystems between 2^31 and 2^32-1 blocks.

Thanks to Valerie Aurora Henson for pointing out the problems in
lib/ext2fs/alloc_tables.c, which led me to do a "make gcc-wall" scan
over the source tree.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/lib/ext2fs/inode.c b/lib/ext2fs/inode.c
index b75cb77..b3ac9f2 100644
--- a/lib/ext2fs/inode.c
+++ b/lib/ext2fs/inode.c
@@ -17,6 +17,7 @@
 #if HAVE_ERRNO_H
 #include <errno.h>
 #endif
+#include <time.h>
 #if HAVE_SYS_STAT_H
 #include <sys/stat.h>
 #endif