[nrf fromtree] samples: net: prometheus: Add netif dependency

There's no point building the sample for platforms that do not support
networking, hence add netif dependency to reduce the CI execution
scope.

Signed-off-by: Robert Lubos <[email protected]>
(cherry picked from commit 3df9b5f)

Author: rlubos

samples/net/prometheus/sample.yaml

@@ -3,6 +3,7 @@ sample:
   name: prometheus_client_sample
 common:
   harness: net
+  depends_on: netif
   min_ram: 192
   tags:
     - http