src: hide kMaxDigestMultiplier outside HKDF impl

There is no reason to expose this constant outside of the HKDF
implementation, especially with such a generic name.

Author: tniessen

src/crypto/crypto_hkdf.cc

@@ -17,6 +17,8 @@ using v8::Uint32;
 using v8::Value;
 
 namespace crypto {
+static constexpr size_t kMaxDigestMultiplier = 255;
+
 HKDFConfig::HKDFConfig(HKDFConfig&& other) noexcept
     : mode(other.mode),
       length(other.length),

src/crypto/crypto_hkdf.h

@@ -11,8 +11,6 @@
 
 namespace node {
 namespace crypto {
-static constexpr size_t kMaxDigestMultiplier = 255;
-
 struct HKDFConfig final : public MemoryRetainer {
   CryptoJobMode mode;
   size_t length;