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

Fix compiler warning:
trx/trx0purge.c: In function 'trx_purge_add_update_undo_to_history':
trx/trx0purge.c:307:15: error: variable 'seg_header' set but not used [-Werror=unused-but-set-variable]

storage/innodb_plugin/trx/trx0purge.c

@@ -304,7 +304,9 @@ trx_purge_add_update_undo_to_history(
 	trx_undo_t*	undo;
 	trx_rseg_t*	rseg;
 	trx_rsegf_t*	rseg_header;
+#ifdef UNIV_DEBUG
 	trx_usegf_t*	seg_header;
+#endif /* UNIV_DEBUG */
 	trx_ulogf_t*	undo_header;
 	ulint		hist_size;
 
@@ -320,7 +322,9 @@ trx_purge_add_update_undo_to_history(
 				    rseg->page_no, mtr);
 
 	undo_header = undo_page + undo->hdr_offset;
+#ifdef UNIV_DEBUG
 	seg_header  = undo_page + TRX_UNDO_SEG_HDR;
+#endif /* UNIV_DEBUG */
 
 	if (undo->state != TRX_UNDO_CACHED) {
 		/* The undo log segment will not be reused */