Fix compile on RHEL4/SLES10

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/helpers.c b/helpers.c
index c695331..377dd02 100644
--- a/helpers.c
+++ b/helpers.c
@@ -6,6 +6,7 @@
 #include <unistd.h>
 
 #include "compiler/compiler.h"
+#include "arch/arch.h"
 #include "os/os.h"
 
 #ifndef __NR_fallocate