tcp: fix ports

inline is only support in C99 and newer. To support also C89, use
__inline instead as suggested by dim.

Reported by: eduardo
Reviewed by: rscheff, markj, dim, imp
Tested by: eduardo
Fixes: a8b70cf26030 ("netpfil: Use accessor functions and named constants for all tcphdr flags")
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D43231