Add empty font_lock() for bootloaders to fix builds.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@30817 a1c6a512-1295-4272-9138-f99709370657
diff --git a/firmware/font.c b/firmware/font.c
index 6a9a683..f1cddb0 100644
--- a/firmware/font.c
+++ b/firmware/font.c
@@ -965,6 +965,12 @@
 {
 }
 
+void font_lock(int font_id, bool lock)
+{
+    (void)font_id;
+    (void)lock;
+}
+
 /*
  * Bootloader only supports the built-in sysfont.
  */