Do buildtests on our public header files with C++ as well
Do buildtests on our public header files with C++ as well

This ensures that we don't mistakenly use C++ keywords anywhere public.

Related to #8313

Reviewed-by: Paul Dale <bc4a6b4cd7a8395c734c8eb5aef04a9c30188c85@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8325)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl