Really fix build failure if exclude-kernfs option is not available

Fixes #218, for real this time.

src/dir.h

@@ -107,16 +107,13 @@ extern int (*dir_process)(void);
 
 /* Scanning a live directory */
 extern int dir_scan_smfs;
+extern int exclude_kernfs;
 void dir_scan_init(const char *path);
 
 /* Importing a file */
 extern int dir_import_active;
 int dir_import_init(const char *fn);
 
-#if HAVE_LINUX_MAGIC_H && HAVE_SYS_STATFS_H && HAVE_STATFS
-extern int exclude_kernfs;
-#endif
-
 
 /* The currently configured output functions. */
 extern struct dir_output dir_output;