cmake remove circular linking and reorganize

- px4_add_module now requires MAIN
 - px4_add_library doesn't automatically link

src/modules/load_mon/CMakeLists.txt

@@ -38,6 +38,5 @@ px4_add_module(
 	SRCS
 		load_mon.cpp
 	DEPENDS
-		platforms__common
 	)
-# vim: set noet ft=cmake fenc=utf-8 ff=unix :
+

src/modules/load_mon/load_mon.cpp

@@ -53,7 +53,7 @@
 
 #include <systemlib/systemlib.h>
 #include <systemlib/cpuload.h>
-#include <systemlib/perf_counter.h>
+#include <perf/perf_counter.h>
 
 #include <uORB/uORB.h>
 #include <uORB/topics/cpuload.h>