Rename function to avoid ambiguity on platforms where va_list is void *.

Index: src/lib/barrier/ProtocolUtil.cpp
--- src/lib/barrier/ProtocolUtil.cpp.orig
+++ src/lib/barrier/ProtocolUtil.cpp
@@ -82,7 +82,7 @@ ProtocolUtil::vwritef(barrier::IStream* stream,
 
     // fill buffer
     UInt8* buffer = new UInt8[size];
-    writef(buffer, fmt, args);
+    writef_(buffer, fmt, args);
 
     try {
         // write buffer
@@ -349,7 +349,7 @@ ProtocolUtil::getLength(const char* fmt, va_list args)
 }
 
 void
-ProtocolUtil::writef(void* buffer, const char* fmt, va_list args)
+ProtocolUtil::writef_(void* buffer, const char* fmt, va_list args)
 {
     UInt8* dst = static_cast<UInt8*>(buffer);