community/seahorse: fix build with gcc 14

Fixes rebuild errors seen with gcc 14.

• Avoid C99 incompatibility around seahorse_pkcs11_backend_initialize
• v43.0 Fails to Build With Clang >= 15 Due to Default '-Werror=int-conversion'

For the -Wint-conversion error patch, the Gentoo version has another hunk needed to build to pass, the upstream link is included for reference.