QUIC: fixed bulding perl module by reducing header pollution. The ngx_http_perl_module module doesn't have a notion of including additional search paths through --with-cc-opt, which results in compile error incomplete type 'enum ssl_encryption_level_t' when building nginx without QUIC support. The enum is visible from quic event headers and eventually pollutes ngx_core.h. The fix is to limit including headers to compile units that are real consumers.