patch 8.2.1048: build failure without the eval feature

Problem:    Build failure without the eval feature.
Solution:   Add dummy typedef.
diff --git a/src/structs.h b/src/structs.h
index 3a9bbd3..0ac4864 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1794,6 +1794,10 @@
 {
     int	    dummy;
 } scriptitem_T;
+typedef struct
+{
+    int	    dummy;
+} evalarg_T;
 #endif
 
 // Struct passed between functions dealing with function call execution.