src/platforms/common: move to platforms/common

Script to update include paths:
for i in $(grep -rl 'include <px4_work_queue' src platforms); do sed -i 's/#include <px4_work_queue/#include <px4_platform_common\/px4_work_queue/' $i; done

src/drivers/camera_trigger/camera_trigger.cpp

@@ -50,7 +50,7 @@
 #include <poll.h>
 #include <mathlib/mathlib.h>
 #include <matrix/math.hpp>
-#include <px4_work_queue/ScheduledWorkItem.hpp>
+#include <px4_platform_common/px4_work_queue/ScheduledWorkItem.hpp>
 #include <systemlib/err.h>
 #include <parameters/param.h>
 #include <systemlib/mavlink_log.h>