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/statistics.c

@@ -23,11 +23,11 @@
  * @endverbatim
  */
 
-#include <statistics.h>
+#include <maxscale/statistics.h>
 #include <maxscale/alloc.h>
-#include <maxconfig.h>
+#include <maxscale/maxconfig.h>
 #include <string.h>
-#include <platform.h>
+#include <maxscale/platform.h>
 
 static int thread_count = 0;
 static bool stats_initialized = false;