src/include/pcp/platform_defs.h.in: add a couple of headers for OpenBSD 6.7
src/include/pcp/platform_defs.h.in: add a couple of headers for OpenBSD 6.7

- <inttypes.h> for PRI?64
- <fcntl> for O_CLOEXEC, so we don't #define it again in this file

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp