Remove 'using namespace std' from SerialTypes.h

This unfortunately cuases a ton of flow-down changes because a lot of other
code was depending on that definition existing. This has a fairly large chance
to break builds of external plugins, considering how many internal ones it broke.

src/iosource/pcap/Source.cc

@@ -308,7 +308,7 @@ void PcapSource::Statistics(Stats* s)
 
 void PcapSource::PcapError(const char* where)
 	{
-	string location;
+	std::string location;
 
 	if ( where )
 		location = fmt(" (%s)", where);