hmac.[hc]: move dtls_hmac_storage_init() to .c file

For platforms other than Contiki, dtls_hmac_storage_init() was
defined inline in hmac.h which cluttered the interface.

Change-Id: I9eb1f0b6a299ef3b60560b3d529dde39d610cd33
diff --git a/hmac.c b/hmac.c
index e16eca5..1b8b450 100644
--- a/hmac.c
+++ b/hmac.c
@@ -42,6 +42,10 @@
   free(ctx);
 }
 
+void
+dtls_hmac_storage_init(void) {
+}
+
 #else /* WITH_CONTIKI */
 #include "memb.h"
 MEMB(hmac_context_storage, dtls_hmac_context_t, DTLS_HASH_MAX);
diff --git a/hmac.h b/hmac.h
index c095cfe..3d543c9 100644
--- a/hmac.h
+++ b/hmac.h
@@ -49,12 +49,7 @@
 }
 #endif /* WITH_SHA256 */
 
-#ifndef WITH_CONTIKI
-static inline void dtls_hmac_storage_init(void)
-{ }
-#else
 void dtls_hmac_storage_init(void);
-#endif
 
 /**
  * \defgroup HMAC Keyed-Hash Message Authentication Code (HMAC)