vhost-user-rng: fix meson warning

vhost-user-rng produces a new meson warning:

Configuring 50-qemu-gpu.json using configuration
Configuring 50-qemu-virtiofsd.json using configuration
Configuring 50-qemu-rng.json using configuration
../../tools/vhost-user-rng/meson.build:7: WARNING: The variable(s) 'libexecdir' in the input file
'tools/vhost-user-rng/50-qemu-rng.json.in' are not present in the given configuration data.
Configuring 50-edk2-i386-secure.json using configuration
Configuring 50-edk2-x86_64-secure.json using configuration
Configuring 60-edk2-aarch64.json using configuration

Fix this up.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tools/vhost-user-rng/meson.build

@@ -6,5 +6,5 @@ executable('vhost-user-rng', files(
 
 configure_file(input: '50-qemu-rng.json.in',
                output: '50-qemu-rng.json',
-               configuration: config_host,
+               configuration: { 'libexecdir' : get_option('prefix') / get_option('libexecdir') },
                install_dir: qemu_datadir / 'vhost-user')