patch 9.0.1151: build failure

Problem:    Build failure.
Solution:   Add missing part of :interface change.

src/structs.h

@@ -1440,7 +1440,7 @@ typedef enum
     VAR_JOB,		// "v_job" is used
     VAR_CHANNEL,	// "v_channel" is used
     VAR_INSTR,		// "v_instr" is used
-    VAR_CLASS,		// "v_class" is used
+    VAR_CLASS,		// "v_class" is used (also used for interface)
     VAR_OBJECT,		// "v_object" is used
 } vartype_T;
 
@@ -1482,10 +1482,15 @@ typedef struct {
     char_u	*ocm_init;   // allocated
 } ocmember_T;
 
+#define CLASS_INTERFACE 1
+
 // "class_T": used for v_class of typval of VAR_CLASS
+// Also used for an interface (class_flags has CLASS_INTERFACE).
 struct class_S
 {
     char_u	*class_name;		// allocated
+    int		class_flags;		// CLASS_ flags
+
     int		class_refcount;
     int		class_copyID;		// used by garbage collection
 

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1151,
 /**/
     1150,
 /**/