hkdf_sha256: okm is a void *.

Making arbitrary bytes a uchar* is always a bit nasty for the caller.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/crypto/hkdf_sha256/hkdf_sha256.c

@@ -4,7 +4,7 @@
 #include <assert.h>
 #include <string.h>
 
-void hkdf_sha256(unsigned char *okm, size_t okm_size,
+void hkdf_sha256(void *okm, size_t okm_size,
 		 const void *s, size_t ssize,
 		 const void *k, size_t ksize,
 		 const void *info, size_t isize)
@@ -83,7 +83,7 @@ void hkdf_sha256(unsigned char *okm, size_t okm_size,
 
 	while (okm_size > sizeof(t)) {
 		memcpy(okm, &t, sizeof(t));
-		okm += sizeof(t);
+		okm = (char *)okm + sizeof(t);
 		okm_size -= sizeof(t);
 
 		c++;

ccan/crypto/hkdf_sha256/hkdf_sha256.h

@@ -15,7 +15,7 @@
  * @info: pointer to info
  * @isize: the number of bytes pointed to by @info
  */
-void hkdf_sha256(unsigned char *okm, size_t okm_size,
+void hkdf_sha256(void *okm, size_t okm_size,
 		 const void *s, size_t ssize,
 		 const void *k, size_t ksize,
 		 const void *info, size_t isize);