samples: net: gsm_modem: Add GSM mux compile test

Add a compile test for GSM muxing so that we catch possible
issues in the future.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
diff --git a/samples/net/gsm_modem/sample.yaml b/samples/net/gsm_modem/sample.yaml
index 72e00c9..ba76853 100644
--- a/samples/net/gsm_modem/sample.yaml
+++ b/samples/net/gsm_modem/sample.yaml
@@ -9,3 +9,8 @@
   sample.net.ppp.gsm.modem:
     extra_configs:
       - CONFIG_TEST_RANDOM_GENERATOR=y
+  sample.net.ppp.gsm.modem.mux:
+    extra_configs:
+      - CONFIG_TEST_RANDOM_GENERATOR=y
+      - CONFIG_GSM_MUX=y
+      - CONFIG_UART_MUX=y