LibCrypto: Fix incorrectly constexpr variable

Problem:
- Clang ToT reports an error because `digest_size` cannot be evaluated
  at compile-time.

Solution:
- Change from using the member function to the `static` shadow of the
  NTTP.

Userland/Libraries/LibCrypto/PK/Code/EMSA_PSS.h

@@ -33,7 +33,7 @@ public:
         auto& hash_fn = this->hasher();
         hash_fn.update(in);
         auto message_hash = hash_fn.digest();
-        constexpr auto hash_length = hash_fn.DigestSize;
+        constexpr auto hash_length = HashFunction::DigestSize;
         auto em_length = (em_bits + 7) / 8;
         u8 salt[SaltLength];