Add header guards

src/cheat.h

@@ -1,3 +1,6 @@
+#ifndef _FCEU_CHEAT_H
+#define _FCEU_CHEAT_H
+
 void FCEU_CheatResetRAM(void);
 void FCEU_CheatAddRAM(int s, uint32 A, uint8 *p);
 
@@ -5,3 +8,5 @@ void FCEU_LoadGameCheats(FILE *override);
 void FCEU_FlushGameCheats(FILE *override, int nosave);
 void FCEU_ApplyPeriodicCheats(void);
 void FCEU_PowerCheats(void);
+
+#endif