(partially) Fix Bug#55227 Fix compiler warnings in innodb with gcc 4.6

Fix compiler warning:
trx/trx0purge.c: In function 'trx_purge_rseg_get_next_history_log':
trx/trx0purge.c:660:15: error: variable 'seg_hdr' set but not used [-Werror=unused-but-set-variable]

storage/innodb_plugin/trx/trx0purge.c

@@ -657,7 +657,6 @@ trx_purge_rseg_get_next_history_log(
 {
 	page_t*		undo_page;
 	trx_ulogf_t*	log_hdr;
-	trx_usegf_t*	seg_hdr;
 	fil_addr_t	prev_log_addr;
 	trx_id_t	trx_no;
 	ibool		del_marks;
@@ -678,7 +677,6 @@ trx_purge_rseg_get_next_history_log(
 	undo_page = trx_undo_page_get_s_latched(rseg->space, rseg->zip_size,
 						rseg->last_page_no, &mtr);
 	log_hdr = undo_page + rseg->last_offset;
-	seg_hdr = undo_page + TRX_UNDO_SEG_HDR;
 
 	/* Increase the purge page count by one for every handled log */