Fix comparison between pos and std::string::npos

std::string::npos is defined as the unsigned type size_t but given a value of -1,
resulting in a warning when trying to compare an unsigned int with a size_t.

Fix this by declaring pos as a size_t.

src/UpdaterOptions.cpp

@@ -43,7 +43,7 @@ UpdateInstaller::Mode stringToMode(const std::string& modeStr)
 
 void UpdaterOptions::parseOldFormatArg(const std::string& arg, std::string* key, std::string* value)
 {
-	unsigned int pos = arg.find('=');
+	size_t pos = arg.find('=');
 	if (pos != std::string::npos)
 	{
 		*key = arg.substr(0,pos);