appletlib: fix set-but-unused warning

When you build with FEATURE_INSTALLER disabled, you get a build
warning like so:

libbb/appletlib.c: In function 'busybox_main':
libbb/appletlib.c:691:7: warning: variable 'use_symbolic_links' set but
                                  not used [-Wunused-but-set-variable]
   int use_symbolic_links;
       ^

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

libbb/appletlib.c

@@ -606,7 +606,11 @@ static void install_links(const char *busybox, int use_symbolic_links,
 	}
 }
 # else
-#  define install_links(x,y,z) ((void)0)
+static void install_links(const char *busybox UNUSED_PARAM,
+		int use_symbolic_links UNUSED_PARAM,
+		char *custom_install_dir UNUSED_PARAM)
+{
+}
 # endif
 
 /* If we were called as "busybox..." */