sysutils/flowgger: Fix build with LibreSSL 2.5.5
The build of the openssl-sys-0.9.12 crate fails with:
This crate is only compatible with OpenSSL 1.0.1, 1.0.2, and 1.1.0, or LibreSSL 2.5.0, 2.5.1, 2.5.2, 2.5.3, and 2.5.4, but a different version of OpenSSL was found. The build is now aborting due to this version mismatch.
Update the openssl crates to 0.9.15 (and also its dependencies).
While here also set LICENSE=MPL20