patch 8.1.1267: cannot check if GPM mouse support is working

Problem:    Cannot check if GPM mouse support is working.
Solution:   Add the "mouse_gpm_enable" feature.
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index c12e69e..bc18daa 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -10532,6 +10532,7 @@
 mouse			Compiled with support mouse.
 mouse_dec		Compiled with support for Dec terminal mouse.
 mouse_gpm		Compiled with support for gpm (Linux console mouse)
+mouse_gpm_enabled	GPM mouse is working
 mouse_netterm		Compiled with support for netterm mouse.
 mouse_pterm		Compiled with support for qnx pterm mouse.
 mouse_sysmouse		Compiled with support for sysmouse (*BSD console mouse)