Enable feature reqwest/native-tls-vendored when native-tls and static is enabled

[NobodyXu]

Additionally, I've noticed that I cannot cross-compile from the -gnu build host to a -musl target due to OpenSSL with native-tls, yet I can do this when the feature has reqwest/native-tls-vendored, which will build from source to support cross-compilation.

I can enable native-tls-vendored whenever native-tls is enabled.

Given that they are likely using the same version of openssl, enabling vendored for one would fix the issue.

Originally posted by @NobodyXu in #1688 (comment)

Might be relevant with the static feature if that condition can be used? Otherwise AFAIK it requires users that want to dynamic link to use OPENSSL_NO_VENDOR=1.

Orignally posted by @polarathene in #1688 (comment)