Remove #define loop from util.h

Replace the loop macro with while (true). The #define caused
problems for Qt.

src/bitcoinrpc.cpp

@@ -414,7 +414,7 @@ int ReadHTTPStatus(std::basic_istream<char>& stream, int &proto)
 int ReadHTTPHeaders(std::basic_istream<char>& stream, map<string, string>& mapHeadersRet)
 {
     int nLen = 0;
-    loop
+    while (true)
     {
         string str;
         std::getline(stream, str);