Make KEY_SIZE a compile-time constant

Github-Pull: #9785
Rebased-From: 914fad155d

src/httprpc.cpp

@@ -112,7 +112,7 @@ static bool multiUserAuthorized(std::string strUserPass)
             std::string strSalt = vFields[1];
             std::string strHash = vFields[2];
 
-            unsigned int KEY_SIZE = 32;
+            static const unsigned int KEY_SIZE = 32;
             unsigned char out[KEY_SIZE];
 
             CHMAC_SHA256(reinterpret_cast<const unsigned char*>(strSalt.c_str()), strSalt.size()).Write(reinterpret_cast<const unsigned char*>(strPass.c_str()), strPass.size()).Finalize(out);