Work around GCC6

This problem is really annoying. Left shifting a value does not require
it to be positive in C. However, some specifications define that as
undefined behaviour.

Nevertheless, as used in this case it is perfectly fine and widely used
construct.

Closes #230, #378

src/typedefs.h

@@ -161,7 +161,7 @@ typedef enum {
 	CHANGE_WARN_CHANGED_EXT        = 1 << 3,
 	CHANGE_WARN_UNSAVED_META       = 1 << 4,
 	CHANGE_WARN_NO_WRITE_PERM_DEST_DIR  = 1 << 5,
-	CHANGE_ERROR_MASK              = (~0) << 8, /* the values below are fatal errors */
+	CHANGE_ERROR_MASK              = (~0U) << 8, /* the values below are fatal errors */
 	CHANGE_NO_READ_PERM            = 1 << 8,
 	CHANGE_NO_WRITE_PERM_DIR       = 1 << 9,
 	CHANGE_NO_DEST_DIR             = 1 << 10,