Update ntp_parser.y and keyword-gen.c to avoid dependency on OpenSSL headers

These files include ntp.h, which includes nts.h, which needs openssl/ssl.h. The means both the host (eg, the bison/keyword binaries that get compiled for the host platform) and main (the rest of NTPsec that gets compiled for the target platform) variants of the build need OpenSSL, even though the host part has no direct need for OpenSSL itself.

We need to reorganize the headers to avoid indirectly including openssl/ssl.h in the host variant.

See the thread starting at https://lists.ntpsec.org/pipermail/devel/2019-February/007622.html for more information.

The work-around to get CI going is at c3121f49