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/src/evalfunc.c b/src/evalfunc.c
index 7b6785c..a2be7b9 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -6810,6 +6810,10 @@
 	else if (STRICMP(name, "netbeans_enabled") == 0)
 	    n = netbeans_active();
 #endif
+#ifdef FEAT_MOUSE_GPM
+	else if (STRICMP(name, "mouse_gpm_enabled") == 0)
+	    n = gpm_enabled();
+#endif
 #if defined(FEAT_TERMINAL) && defined(MSWIN)
 	else if (STRICMP(name, "terminal") == 0)
 	    n = terminal_enabled();