emulators/virtualbox-ose{,-nox11}{,-70,-71,-72,-legacy}: Fix build with ftp/curl 8.16.0

kBuild: Compiling VBoxRT - /wrkdirs/usr/ports/emulators/virtualbox-ose-72/work/VirtualBox-7.2.0/src/VBox/Runtime/generic/http-curl.cpp
/wrkdirs/usr/ports/emulators/virtualbox-ose-72/work/VirtualBox-7.2.0/src/VBox/Runtime/generic/http-curl.cpp:702:27: error: assigning to 'curl_proxytype' from incompatible type 'long'

702 |     pThis->enmProxyType = CURLPROXY_HTTP;
    |                           ^~~~~~~~~~~~~~

/usr/local/include/curl/curl.h:791:35: note: expanded from macro 'CURLPROXY_HTTP'

791 | #define CURLPROXY_HTTP            0L /* added in 7.10, new in 7.19.4 default is
    |                                   ^~

Upstream bug report:
https://github.com/curl/curl/issues/18054#issuecomment-3354156324

PR: 289924