linux-user: avoid -Werror=int-in-bool-context

linux-user: avoid -Werror=int-in-bool-context

linux-user is failing to compile on Fedora 43:

../linux-user/strace.c:57:66: error: enum constant in boolean context [-Werror=int-in-bool-context]
   57 | #define FLAG_BASIC(V, M, N)      { V, M | QEMU_BUILD_BUG_ON_ZERO(!(M)), N }

The warning does not seem to be too useful and we could even disable it,
but the workaround is simple in this case.

Cc: qemu-stable@nongnu.org
Cc: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>