rpcgen: tests: Include stdint.h in test_demo.c

rpcgen: tests: Include stdint.h in test_demo.c

Since header file structure is a bit different on MacOS, it
doesn't get uint64_t type declaration and thus test_demo.c must
include it explicitly. This is proper solution anyway, because on
Linux we're apparently relying on the header file sneaking
through some other include.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/619
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>