fix syntax err from misplaced ifdef

Author: paul-szczepanek-arm

drivers/SPI.h

@@ -326,9 +326,9 @@ class SPI : private NonCopyable<SPI> {
      *  Implemented in order to avoid duplicate locking and boost performance.
      */
     void _acquire(void);
-};
 
 #endif //!defined(DOXYGEN_ONLY)
+};
 
 } // namespace mbed