Added include guards to timer header file

includes/timer.h

@@ -1,7 +1,8 @@
 #ifdef __cplusplus
 extern "C" {
 #endif
-
+#ifndef _TIMER_H
+#define _TIMER_H
 /* This file defines a simple timer struct, and methods to initialize it,
 and keep track of time elapsed.
 It was copied from https://github.com/raysan5/raylib/wiki/Frequently-Asked-Questions#how-do-i-make-a-timer */
@@ -30,6 +31,7 @@ double timer_get_elapsed(Timer timer)
     return GetTime() - timer.start_time;
 }
 
+#endif
 #ifdef __cplusplus
 }
 #endif