patch 8.1.1265: when GPM mouse support is enabled double clicks do not work

Problem:    When GPM mouse support is enabled double clicks in xterm do not
            work.
Solution:   Use KS_GPM_MOUSE for GPM mouse events.
diff --git a/src/os_unix.c b/src/os_unix.c
index ba5d386..9f8a7b1 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -3794,7 +3794,10 @@
 	    && !gui.in_use
 #  endif
 	    )
-	set_mouse_termcode(KS_MOUSE, (char_u *)IF_EB("\033MG", ESC_STR "MG"));
+	set_mouse_termcode(KS_GPM_MOUSE,
+				      (char_u *)IF_EB("\033MG", ESC_STR "MG"));
+    else
+	del_mouse_termcode(KS_GPM_MOUSE);
 # endif
 
 # ifdef FEAT_SYSMOUSE
@@ -7065,7 +7068,8 @@
 	Gpm_Close();
 }
 
-/* Reads gpm event and adds special keys to input buf. Returns length of
+/*
+ * Reads gpm event and adds special keys to input buf. Returns length of
  * generated key sequence.
  * This function is styled after gui_send_mouse_event().
  */