Added guards to PI define

Added guards to not redefine it if the user is using it with raylib.h
also added an 'f' at the end of the define to keep compliant with raylib's PI define

src/easings.h

@@ -91,7 +91,10 @@
 #endif
 
 #include <math.h>       // Required for: sin(), cos(), sqrt(), pow()
-#define PI 3.14159265358979323846 //Required as PI is not always defined in math.h
+
+#ifndef PI
+    #define PI 3.14159265358979323846f //Required as PI is not always defined in math.h
+#endif
 
 #ifdef __cplusplus
 extern "C" {            // Prevents name mangling of functions