scpi_vxi: Unbreak MacOS build (vxi.h inclusion order).

Undo the #include statement order change of commit 7414fb55cb. It's
essential on MacOS to include vxi.h early to keep related u_long et al
data types available. See commit d40b8557a9 for a discussion.

Also drop the inclusion of rpc/rpc.h from scpi_vxi.c because it is not
used in that source file, instead is a vxi.h dependency and is included
there.

[ gsi: rephrase commit message ]

src/scpi/scpi_vxi.c

@@ -21,12 +21,12 @@
  */
 
 #include <config.h>
-#include <rpc/rpc.h>
+/* vxi.h must the first #include after config.h See commit d40b8557. */
+#include "vxi.h"
 #include <string.h>
 #include <libsigrok/libsigrok.h>
 #include "libsigrok-internal.h"
 #include "scpi.h"
-#include "vxi.h"
 
 #define LOG_PREFIX "scpi_vxi"
 #define VXI_DEFAULT_TIMEOUT_MS 2000