test/common: skip unittest_back_trace with mingw-gcc

test/common: skip unittest_back_trace with mingw-gcc

We're getting an ICE when trying to compile this test using
mingw-gcc and recent Boost versions. Note that mingw-llvm works fine.

    during IPA pass: inline
    /mnt/data/workspace/ceph.pr/src/test/common/test_back_trace.cc:44:1:
    internal compiler error: Segmentation fault
       44 | }
          | ^
    0x7f9c4a86c51f ???
            ./signal/../sysdeps/unix/sysv/linux/x86_64/libc_sigaction.c:0
    0x7f9c4a853d8f __libc_start_call_main
            ../sysdeps/x86/libc-start.c:58
    0x7f9c4a853e3f __libc_start_main_impl
            ../sysdeps/nptl/libc_start_call_main.h:392

For now, we'll just skip the test when using mingw-gcc. Note
that we're planing to switch to mingw-llvm anyway:
https://github.com/ceph/ceph/pull/51197

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>