SDL_endian.h: Use endian.h for OpenBSD.

Patch from OpenBSD CVS, authored by Donovan Watteau.

include/SDL_endian.h

@@ -42,7 +42,10 @@
 #ifdef __linux__
 #include <endian.h>
 #define SDL_BYTEORDER  __BYTE_ORDER
-#else /* __linux__ */
+#elif defined(__OpenBSD__)
+#include <endian.h>
+#define SDL_BYTEORDER  BYTE_ORDER
+#else
 #if defined(__hppa__) || \
     defined(__m68k__) || defined(mc68000) || defined(_M_M68K) || \
     (defined(__MIPS__) && defined(__MIPSEB__)) || \