hw/boards: make it safe to include for linux-user

Make it safe to include hw/boards.h in exec.c
for linux-user configurations.
We don't need any of its contents though.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

include/hw/boards.h

@@ -3,6 +3,8 @@
 #ifndef HW_BOARDS_H
 #define HW_BOARDS_H
 
+#if !defined(CONFIG_USER_ONLY)
+
 #include "qemu/typedefs.h"
 #include "sysemu/blockdev.h"
 #include "sysemu/accel.h"
@@ -154,3 +156,5 @@ struct MachineState {
 };
 
 #endif
+
+#endif