Fix header writing

Seeking from the start of an empty ofstream can yield undefined behavior, for certain paks it corrupts the stream. Just pad out the header with the new method that was added in f96c711. This fixes ui_sdk.rpak coming out corrupt.

src/logic/pakfile.cpp

@@ -743,7 +743,7 @@ void CPakFileBuilder::BuildFromMap(const string& mapPath)
 
 	// write a placeholder header so we can come back and complete it
 	// when we have all the info
-	out.Seek(pakVersion >= 8 ? 0x80 : 0x58);
+	out.Pad(pakVersion >= 8 ? 0x80 : 0x58);
 
 	const char* streamFileMandatory = nullptr;