basic-forward: Add WaitFlags forward decl

src/basic/basic-forward.h

@@ -94,6 +94,7 @@ typedef enum TimestampStyle TimestampStyle;
 typedef enum UnitActiveState UnitActiveState;
 typedef enum UnitDependency UnitDependency;
 typedef enum UnitType UnitType;
+typedef enum WaitFlags WaitFlags;
 
 typedef struct Hashmap Hashmap;
 typedef struct HashmapBase HashmapBase;