hwplugin.c: Add missing config.h #include.

This fixes a bug where no LA would be found or displayed in lists such
as 'sigrok-cli -H' or 'sigrok-cli -D'.

Thanks Forrest Voight for spotting the bug.

hwplugin.c

@@ -24,6 +24,7 @@
 #include <string.h>
 #include <glib.h>
 #include <sigrok.h>
+#include "config.h"
 
 source_callback_add source_cb_add = NULL;
 source_callback_remove source_cb_remove = NULL;