Move headers from server/include to include/maxscale

- Headers now to be included as <maxscale/xyz.h>
- First step, no cleanup of headers has been made. Only moving
  from one place to another + necessary modifications.

server/modules/routing/avro/avro_index.c

@@ -33,7 +33,7 @@
  */
 
 #include <avrorouter.h>
-#include <skygw_debug.h>
+#include <maxscale/skygw_debug.h>
 #include <glob.h>
 
 void* safe_key_free(void *data);