[Maintenance] Add include guards to ReC98.h

They do make everything so much simpler, after all. Especially now that
th01/formats/grz.cpp should compile as a freestanding translation unit,
as part of the .GRZ viewer.

Part of P0082, funded by Ember2528.

ReC98.h

@@ -3,6 +3,9 @@
  * Main include file
  */
 
+#ifndef REC98_H
+#define REC98_H
+
 #include <master.h>
 #include <stddef.h>
 #include "platform.h"
@@ -137,3 +140,5 @@ extern dots8_t *VRAM_PLANE_E;
 
 void pascal vram_planes_set(void);
 // -----------------
+
+#endif /* REC98_H */

th01/formats/grz.cpp

@@ -5,7 +5,6 @@ extern "C" {
 #include <stddef.h>
 #include <string.h>
 
-#include <master.h>
 #include "ReC98.h"
 #include "th01/hardware/graph.h"
 #include "th01/formats/grz.h"