mmc-utils: remove unused #includes

I didn't find any functions used for which the documentation specifies to
use (at least) one of the dropped headers.

The only Linux specific header <linux/fs.h> is needed for BLKGETSIZE,
document that.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Chris Ball <chris@printf.net>
diff --git a/mmc_cmds.c b/mmc_cmds.c
index cec947d..038dbd4 100644
--- a/mmc_cmds.c
+++ b/mmc_cmds.c
@@ -22,17 +22,13 @@
 #include <string.h>
 #include <sys/ioctl.h>
 #include <sys/types.h>
-#include <dirent.h>
 #include <sys/stat.h>
 #include <unistd.h>
 #include <fcntl.h>
-#include <libgen.h>
-#include <limits.h>
-#include <ctype.h>
 #include <errno.h>
 #include <stdint.h>
 #include <assert.h>
-#include <linux/fs.h>
+#include <linux/fs.h> /* for BLKGETSIZE */
 
 #include "mmc.h"
 #include "mmc_cmds.h"