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/core/spinlock.c

@@ -23,10 +23,10 @@
  * @endverbatim
  */
 
-#include <spinlock.h>
-#include <atomic.h>
+#include <maxscale/spinlock.h>
+#include <maxscale/atomic.h>
 #include <time.h>
-#include <skygw_debug.h>
+#include <maxscale/skygw_debug.h>
 
 /**
  * Initialise a spinlock.