LibCrypto: Exclude class_name() methods from the Kernel

These are only used by Userland and contain infallible String
allocations, so let's just ifdef them out of the Kernel.

Userland/Libraries/LibCrypto/Hash/MD5.h

@@ -6,10 +6,13 @@
 
 #pragma once
 
-#include <AK/String.h>
 #include <AK/Types.h>
 #include <LibCrypto/Hash/HashFunction.h>
 
+#ifndef KERNEL
+#    include <AK/String.h>
+#endif
+
 namespace Crypto {
 namespace Hash {
 
@@ -53,7 +56,12 @@ public:
     virtual DigestType digest() override;
     virtual DigestType peek() override;
 
-    virtual String class_name() const override { return "MD5"; }
+#ifndef KERNEL
+    virtual String class_name() const override
+    {
+        return "MD5";
+    }
+#endif
 
     inline static DigestType hash(const u8* data, size_t length)
     {