fix missing newline in thermo output.

src/thermo.cpp

@@ -315,6 +315,7 @@ void Thermo::header()
 
   std::string hdr;
   for (int i = 0; i < nfield; i++) hdr +=  keyword[i] + std::string(" ");
+  hdr += "\n";
   if (me == 0) utils::logmesg(lmp,hdr);
 }