patch 8.2.0833: mapping <C-bslash> doesn't work in the GUI

Problem:    Mapping <C-bslash> doesn't work in the GUI.
Solution:   Reset seenModifyOtherKeys when starting the GUI. (closes #6150)

src/gui.c

@@ -803,6 +803,8 @@ gui_init(void)
 	if (!p_beval)
 	    gui_mch_disable_beval_area(balloonEval);
 #endif
+	// In case the terminal was used before ":gui".
+	seenModifyOtherKeys = FALSE;
 
 #if defined(FEAT_XIM) && defined(FEAT_GUI_GTK)
 	if (!im_xim_isvalid_imactivate())

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    833,
 /**/
     832,
 /**/