tcphdr.h: special-case endian.h for mac/bsd

this *should* work

if it doesn't, add this right down after the endian block:
````
#ifndef __BYTE_ORDER
#define __BYTE_ORDER BYTE_ORDER
#endif
#ifndef __LITTLE_ENDIAN
#define __LITTLE_ENDIAN LITTLE_ENDIAN
#define __BIG_ENDIAN BIG_ENDIAN
#endif
````