put implementation header first for all remaining main folder source files

src/variable.cpp

@@ -11,13 +11,13 @@
    See the README file in the top-level LAMMPS directory.
 ------------------------------------------------------------------------- */
 
+#include "variable.h"
 #include <mpi.h>
 #include <cmath>
 #include <cstdlib>
 #include <cstring>
 #include <cctype>
 #include <unistd.h>
-#include "variable.h"
 #include "universe.h"
 #include "atom.h"
 #include "update.h"