janitor: add guards to headers

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/hw/flash.h b/hw/flash.h
index d790f3c..bda2158 100644
--- a/hw/flash.h
+++ b/hw/flash.h
@@ -1,3 +1,6 @@
+#ifndef HW_FLASH_H
+#define HW_FLASH_H 1
+
 /* NOR flash devices */
 
 #include "memory.h"
@@ -57,3 +60,5 @@
 uint8_t ecc_digest(ECCState *s, uint8_t sample);
 void ecc_reset(ECCState *s);
 extern VMStateDescription vmstate_ecc_state;
+
+#endif