[build] Avoid spurious unused-but-set-variable warnings in gcc 4.6

The __table_entries() construction seems to trigger a false positive
warning in gcc 4.6 relating to variables which are set but never
used.  Add __attribute__((unused)) to inhibit this warning.

Reported-by: Ralph Giles <giles@thaumas.net>
Tested-by: Ralph Giles <giles@thaumas.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/tables.h

  */
 #define __table_entries( table, idx ) ( {				\
 	static __table_type ( table ) __table_entries[0]		\
-		__table_entry ( table, idx ); 				\
+		__table_entry ( table, idx ) 				\
+		__attribute__ (( unused ));				\
 	__table_entries; } )
 
 /**