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)
diff --git a/src/gui.c b/src/gui.c
index 93b8bb6..a7216fc 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -803,6 +803,8 @@
 	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())