util.h: include missing.h, for struct file_handle

This breaks udev-builtin-btrfs.c, which reinvents some of missing.h,
so use missing.h there too.

[zj: removed #include "config.h" and wrapped #include <linux/btrfs.h>
     in ifdef HAVE_LINUX_BTRFS_H as discussed on the mailing list.]

src/shared/util.h

@@ -84,6 +84,7 @@
 #endif
 
 #include "macro.h"
+#include "missing.h"
 #include "time-util.h"
 
 /* What is interpreted as whitespace? */

src/udev/udev-builtin-btrfs.c

@@ -24,15 +24,13 @@
 #include <fcntl.h>
 #include <errno.h>
 #include <sys/ioctl.h>
+#ifdef HAVE_LINUX_BTRFS_H
+#include <linux/btrfs.h>
+#endif
 
+#include "missing.h"
 #include "udev.h"
 
-#define BTRFS_PATH_NAME_MAX 4087
-struct btrfs_ioctl_vol_args {
-        int64_t fd;
-        char name[BTRFS_PATH_NAME_MAX + 1];
-};
-#define BTRFS_IOCTL_MAGIC 0x94
 #define BTRFS_IOC_DEVICES_READY _IOR(BTRFS_IOCTL_MAGIC, 39, struct btrfs_ioctl_vol_args)
 
 static int builtin_btrfs(struct udev_device *dev, int argc, char *argv[], bool test) {