patch 8.1.2170: cannot build without the +termresponse feature

Problem:    Cannot build without the +termresponse feature.
Solution:   Add #ifdef.
diff --git a/src/term.c b/src/term.c
index b287121..ef857a1 100644
--- a/src/term.c
+++ b/src/term.c
@@ -1957,8 +1957,10 @@
 #if defined(UNIX) || defined(VMS)
     term_is_xterm = vim_is_xterm(term);
 #endif
+#ifdef FEAT_TERMRESPONSE
     is_not_xterm = FALSE;
     is_mac_terminal = FALSE;
+#endif
 
 #ifdef FEAT_MOUSE
 # if defined(UNIX) || defined(VMS)