patch 8.2.2983: Vim9: an inline function requires specifying the return type

Problem:    Vim9: an inline function requires specifying the return type.
Solution:   Make the return type optional.
diff --git a/src/userfunc.c b/src/userfunc.c
index 64e815d..f610c02 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -1377,7 +1377,7 @@
 		    goto errret;
 	    }
 	    else
-		fp->uf_ret_type = &t_any;
+		fp->uf_ret_type = &t_unknown;
 	}
 
 	fp->uf_lines = newlines;