patch 9.1.1654: build failure when FEAT_DIFF is not defined

Problem:  build failure when FEAT_DIFF is not defined
          (John Marriott)
Solution: Remove FEAT_DIFF in proto.h; define dummy type for
          diffline_T and diffline_change_T when FEAT_DIFF is not defined

related: #18026
closes: #18046

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/proto.h

@@ -73,9 +73,7 @@ extern int _stricoll(char *a, char *b);
 # include "if_cscope.pro"
 # include "debugger.pro"
 # include "dict.pro"
-# ifdef FEAT_DIFF
 # include "diff.pro"
-# endif
 # include "linematch.pro"
 # include "digraph.pro"
 # include "drawline.pro"

src/structs.h

@@ -3671,6 +3671,9 @@ struct diffline_S
     int bufidx;
     int lineoff;
 };
+#else  // FEAT_DIFF
+typedef void diffline_T;
+typedef void diffline_change_T;
 #endif
 
 #define SNAP_HELP_IDX	0

src/version.c

@@ -724,6 +724,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1654,
 /**/
     1653,
 /**/