meson: fix migration/stress compilation with glibc>=2.30

gettid() was introduced with glibc 2.30.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200828110734.1638685-16-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>