powerpc/microwatt: Make microwatt_get_random_darn() static

Make microwatt_get_random_darn() static, because it can be.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211118004415.1706863-1-mpe@ellerman.id.au

arch/powerpc/platforms/microwatt/rng.c

@@ -14,7 +14,7 @@
 
 #define DARN_ERR 0xFFFFFFFFFFFFFFFFul
 
-int microwatt_get_random_darn(unsigned long *v)
+static int microwatt_get_random_darn(unsigned long *v)
 {
 	unsigned long val;