drivers: entropy: nrf5: add dependency on MULTITHREADING

The driver internally uses semaphores which are only available if
CONFIG_MULTITHREADING is enabled in the build.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>
diff --git a/drivers/entropy/Kconfig.nrf5 b/drivers/entropy/Kconfig.nrf5
index 37f6b0e..92e866a 100644
--- a/drivers/entropy/Kconfig.nrf5
+++ b/drivers/entropy/Kconfig.nrf5
@@ -16,6 +16,7 @@
 	default y
 	depends on !ENTROPY_NRF_FORCE_ALT
 	depends on DT_HAS_NORDIC_NRF_RNG_ENABLED
+	depends on MULTITHREADING # for k_sem
 	select ENTROPY_HAS_DRIVER
 	help
 	  This option enables the RNG peripheral, which is a random number