support/testing: TestXvisor: bump custom opensbi version to 1.6
Since Buildroot commit [1] "update to Bootlin toolchains 2025.08-1",
the TestXvisor fails when building xvisor with gcc 15.1.0.
Since xvisor is now fixed, opensbi 1.5 fail to build with gcc-15 for
the same reasons.
opensbi 1.6 include a fix for C23 enabled compilers [2]
[1] https://gitlab.com/buildroot.org/buildroot/-/commit/947dbc92a20c5acea7882166cae7893e6ea661e1
[2] https://github.com/riscv-software-src/opensbi/commit/d539d64a23bbd64ba324eed595449bcc9c7be0cb
Cc: Julien Olivain <ju.o@free.fr>
Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Julien Olivain <ju.o@free.fr>
Romain Naour