patch 8.1.1392: build failure in tiny version

Problem:    Build failure in tiny version.
Solution:   Define ex_popupclear to ex_ni if not implemented.  Add UNUSED.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index c6e00be..9f127ec 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -397,6 +397,9 @@
 #if !defined(FEAT_X11) || !defined(FEAT_XCLIPBOARD)
 # define ex_xrestore		ex_ni
 #endif
+#if !defined(FEAT_TEXT_PROP)
+# define ex_popupclear		ex_ni
+#endif
 
 /*
  * Declare cmdnames[].