protocol/ArgParser: move struct RangeArg to separate header

src/command/DatabaseCommands.cxx

@@ -26,6 +26,7 @@
 #include "db/Count.hxx"
 #include "db/Selection.hxx"
 #include "CommandError.hxx"
+#include "protocol/RangeArg.hxx"
 #include "client/Client.hxx"
 #include "client/Response.hxx"
 #include "tag/ParseName.hxx"

src/command/QueueCommands.cxx

@@ -21,6 +21,7 @@
 #include "QueueCommands.hxx"
 #include "Request.hxx"
 #include "CommandError.hxx"
+#include "protocol/RangeArg.hxx"
 #include "db/DatabaseQueue.hxx"
 #include "db/Selection.hxx"
 #include "song/Filter.hxx"

src/command/Request.hxx

@@ -22,6 +22,7 @@
 
 #include "check.h"
 #include "protocol/ArgParser.hxx"
+#include "protocol/RangeArg.hxx"
 #include "Chrono.hxx"
 #include "util/ConstBuffer.hxx"