patch 8.0.1711: term_setsize() is not implemented yet

Problem:    Term_setsize() is not implemented yet.
Solution:   Implement it.
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 4b09951..599d61a 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -876,6 +876,7 @@
 # endif
     {"term_setkill",	2, 2, f_term_setkill},
     {"term_setrestore",	2, 2, f_term_setrestore},
+    {"term_setsize",	3, 3, f_term_setsize},
     {"term_start",	1, 2, f_term_start},
     {"term_wait",	1, 2, f_term_wait},
 #endif