tests: mqtt_publisher: enable random numbers

tls_entropy_func() requires an entropy source.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

tests/net/lib/mqtt_publisher/prj_tls.conf

@@ -42,4 +42,6 @@ CONFIG_MBEDTLS_BUILTIN=y
 CONFIG_MBEDTLS_ENABLE_HEAP=y
 CONFIG_MBEDTLS_HEAP_SIZE=30000
 
+# for tls_entropy_func
+CONFIG_TEST_RANDOM_GENERATOR=y
 CONFIG_ZTEST=y