[AVR32] Export clear_page symbol

Add missing EXPORT_SYMBOL(clear_page), allowing ext3 to be compiled
as a module.
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>

arch/avr32/kernel/avr32_ksyms.c

@@ -29,6 +29,7 @@ EXPORT_SYMBOL(__avr32_asr64);
  */
 EXPORT_SYMBOL(memset);
 EXPORT_SYMBOL(memcpy);
+EXPORT_SYMBOL(clear_page);
 
 /*
  * Userspace access stuff.