rpc: fix race against dtrace probe generator

rpc: fix race against dtrace probe generator

The virt_socket_lib library has a dep on dtrace_gen_headers, but
the virprobe.h file (which includes the libvirt_probes.h) is also
used from virnetserverclient.c and virkeepalive.c files which do
not directly depend on virt_socket_lib. Thus it is possible for
the latter files to be built before the libvirt_probes.h file
has had its content written.

Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>