patch 8.1.1097: Motif build fails

Problem:    Motif build fails. (Paul Jolly)
Solution:   Only use gui_mch_maximized() for MS-Windows. (closes #4194)
diff --git a/src/gui.c b/src/gui.c
index 7017066..0e2686d 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -678,7 +678,7 @@
     /* All components of the GUI have been created now */
     gui.shell_created = TRUE;
 
-#ifndef FEAT_GUI_GTK
+#ifdef FEAT_GUI_MSWIN
     // Set the shell size, adjusted for the screen size.  For GTK this only
     // works after the shell has been opened, thus it is further down.
     // If the window is already maximized (e.g. when --windowid is passed in),
@@ -688,6 +688,10 @@
 	gui_set_shellsize(FALSE, TRUE, RESIZE_BOTH);
     else
 	gui_set_shellsize(TRUE, TRUE, RESIZE_BOTH);
+#else
+# ifndef FEAT_GUI_GTK
+    gui_set_shellsize(FALSE, TRUE, RESIZE_BOTH);
+# endif
 #endif
 #if defined(FEAT_GUI_MOTIF) && defined(FEAT_MENU)
     /* Need to set the size of the menubar after all the menus have been