patch 8.2.0792: build failure with small features

Problem:    Build failure with small features.
Solution:   Add #ifdef.
diff --git a/src/popupwin.c b/src/popupwin.c
index 79021c3..cee848a 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -1757,6 +1757,7 @@
     }
 }
 
+#ifdef FEAT_TERMINAL
 /*
  * Return TRUE if there is any popup window with a terminal buffer.
  */
@@ -1775,6 +1776,7 @@
 		return TRUE;
     return FALSE;
 }
+#endif
 
 /*
  * popup_create({text}, {options})