tests/tcg/s390x: Use the SLOF libc headers for the multiarch tests

tests/tcg/s390x: Use the SLOF libc headers for the multiarch tests

Compiling the s390x tests on Fedora, which has the s390x cross-compiler
installed, fails with:

    In file included from qemu/tests/tcg/s390x/console.c:8:
    qemu/tests/tcg/s390x/../../../pc-bios/s390-ccw/sclp.c:11:10: fatal error: string.h: No such file or directory
       11 | #include <string.h>

This is because Fedora does not have a cross-libc. Since console.c
already uses the SLOF libc implementation, add the respective headers
to the include path.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20241216133819.78583-1-iii@linux.ibm.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>