drivers/char/random.c: add missing include <linux/hw_random.h>

Include linux/hw_random.h header file to silence the following
Wmissing-prototypes gcc warning:
drivers/char/random.c:2346:6: warning: no previous prototype for ‘add_hwgenerator_randomness’ [-Wmissing-prototypes]

---
 drivers/char/random.c | 1 +
 1 file changed, 1 insertion(+)

index 0a381897e4a5..02c65dd417ae 100644
@@ -265,6 +265,7 @@
 #include <linux/syscalls.h>
 #include <linux/completion.h>
 #include <linux/uuid.h>
+#include <linux/hw_random.h>
 #include <crypto/chacha20.h>
 
 #include <asm/processor.h>

2.19.1