qemu 1:6.0+dfsg-2expubuntu2: Fail to build against OpenSSL 3.0

Issue discovered after doing a "No-change rebuild" upload to Jammy while working at the liburing2 migration (LP: #1944037). 

Full build log:

https://launchpadlibrarian.net/570888790/buildlog_ubuntu-jammy-amd64.qemu_1%3A6.0+dfsg-2expubuntu3_BUILDING.txt.gz

Failure mode:

/<<BUILDDIR>>/qemu-6.0+dfsg/roms/skiboot/libstb/create-container.c: In function ‘getPublicKeyRaw’:
/<<BUILDDIR>>/qemu-6.0+dfsg/roms/skiboot/libstb/create-container.c:85:17: error: ‘EVP_PKEY_get1_EC_KEY’ is deprecated: Since OpenSSL 3.0 [-Werror=deprecated-declarations]

Also note that:

cc1: all warnings being treated as errors

Upstream skiboot [1] still uses EVP_PKEY_get1_EC_KEY in master, and don't have an open issue about this. To be filed once we setup a reproducer that builds skiboot "standalone", outside of the qemu source tree.

For the moment we have to relax the severity of that deprecation error, likely appending a -Wno-deprecated-declarations somewhere in d/rules.


[1] https://github.com/open-power/skiboot