samples: subsys: random: Correct Mutex define

Correct Mutex ctr_lock defination as the wrong defination lead to
sysworkq task not acquiring this mutex during bt init, which lead to
BLE didn't work as described in issue zephyrproject-rtos#86444

Signed-off-by: Ying Zhang <[email protected]>

subsys/random/random_ctr_drbg.c

@@ -24,7 +24,7 @@
 static const struct device *entropy_dev;
 static const unsigned char drbg_seed[] = CONFIG_CS_CTR_DRBG_PERSONALIZATION;
 static bool ctr_initialised;
-static struct k_mutex ctr_lock;
+static K_MUTEX_DEFINE(ctr_lock);
 
 static mbedtls_ctr_drbg_context ctr_ctx;