skip definition of inline swap16() function in case a macro already exists (#175)

Fixes the build on OpenBSD.

types.h

@@ -63,7 +63,9 @@ static FORCEINLINE uint UintMax(uint a, uint b) { return a > b ? a : b; }
 #define DWORD(x) (*(uint32*)&(x))
 #define XY(x, y) ((y)*64+(x))
 
+#ifndef swap16
 static inline uint16 swap16(uint16 v) { return (v << 8) | (v >> 8); }
+#endif
 
 typedef struct Point16U {
   uint16 x, y;