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/os_unix.c b/src/os_unix.c
index 9f8a7b1..b5627b1 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -7022,7 +7022,7 @@
 }
 #endif /* !HAVE_RENAME */
 
-#ifdef FEAT_MOUSE_GPM
+#if defined(FEAT_MOUSE_GPM) || defined(PROTO)
 /*
  * Initializes connection with gpm (if it isn't already opened)
  * Return 1 if succeeded (or connection already opened), 0 if failed
@@ -7059,12 +7059,21 @@
 }
 
 /*
+ * Returns TRUE if the GPM mouse is enabled.
+ */
+    int
+gpm_enabled(void)
+{
+    return gpm_flag && gpm_fd >= 0;
+}
+
+/*
  * Closes connection to gpm
  */
     static void
 gpm_close(void)
 {
-    if (gpm_flag && gpm_fd >= 0) /* if Open */
+    if (gpm_enabled())
 	Gpm_Close();
 }