CpuUid: moved includes of string and vector

The problem appeared in the travis-ci build when building the SAMR21G18A target. It seems this target lacks support of STL somehow.
If DEVICE_CPUUID is not defined, the string and vector headers would still be included - so on "buggy stl" targets, this would lead to build error

drivers/CpuUid.h

@@ -16,12 +16,12 @@
 #ifndef MBED_CPUUID_H
 #define MBED_CPUUID_H
 
-#include <string>
-#include <vector>
 #include "platform/platform.h"
 
 #if defined(DEVICE_CPUUID) || defined(DOXYGEN_ONLY)
 
+#include <string>
+#include <vector>
 
 namespace mbed {
 /** \addtogroup drivers */