Don't include arch.h, include sysdep.h instead

arch.h is supposed to be a simple configuration file and few have inclusion
guards.  sysdep.h is the proper interface to use.
diff --git a/fetch.c b/fetch.c
index bce949f..a1eb8f4 100644
--- a/fetch.c
+++ b/fetch.c
@@ -22,8 +22,8 @@
 #include <string.h>
 
 #include "fetch.h"
+#include "sysdep.h"
 #include "value.h"
-#include "arch.h"
 #include "type.h"
 
 #ifdef ARCH_HAVE_FETCH_ARG