basic/unit-name: make sure UnitNameFlags is signed

Without that, a check like unit_name_to_instance(...) < 0 would not
have the expected effect.

src/basic/unit-name.h

@@ -13,6 +13,7 @@ typedef enum UnitNameFlags {
         UNIT_NAME_TEMPLATE = 1 << 1, /* Allow foo@.service */
         UNIT_NAME_INSTANCE = 1 << 2, /* Allow foo@bar.service */
         UNIT_NAME_ANY = UNIT_NAME_PLAIN|UNIT_NAME_TEMPLATE|UNIT_NAME_INSTANCE,
+        _UNIT_NAME_INVALID = -1,
 } UnitNameFlags;
 
 bool unit_name_is_valid(const char *n, UnitNameFlags flags) _pure_;