dispatcher: make global data in "nm-dispatcher.c" static

It's not needed outside the source file, and lgtm.com complains
that global variables should have a long name.

  Poor global variable name 'gl'. Prefer longer, descriptive names for
  globals (eg. kMyGlobalConstant, not foo).

src/nm-dispatcher/nm-dispatcher.c

@@ -62,7 +62,7 @@ typedef struct {
     bool shutdown_quitting;
 } GlobalData;
 
-GlobalData gl;
+static GlobalData gl;
 
 typedef struct {
     Request *request;