stubs/meson: Fix qemuutil build when --disable-system

stubs/meson: Fix qemuutil build when --disable-system

Compiling without system, user, tools or guest-agent fails with the
following error message:

./configure --disable-system --disable-user --disable-tools \
--disable-guest-agent

error message:

/usr/bin/ld: libqemuutil.a.p/util_error-report.c.o: in function `error_printf':
/media/liuzhao/data/qemu-cook/build/../util/error-report.c:38: undefined reference to `error_vprintf'
/usr/bin/ld: libqemuutil.a.p/util_error-report.c.o: in function `vreport':
/media/liuzhao/data/qemu-cook/build/../util/error-report.c:215: undefined reference to `error_vprintf'
collect2: error: ld returned 1 exit status

This is because tests/bench and tests/unit both need qemuutil, which
requires error_vprintf stub when system is disabled.

Add error_vprintf stub into stub_ss for all cases other than disabling
system.

Fixes: 3a15604900c4 ("stubs: include stubs only if needed")
Reported-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Message-ID: <20240605152549.1795762-1-zhao1.liu@intel.com>
[Include error-printf.c unconditionally. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>