Fix broken include guard in lpc43xx/cgu.h

[martinling]

Recent versions of GCC catch this mistake with -Wheader-guard:

include/libopencm3/lpc43xx/cgu.h:36: warning: header guard 'LPC43XX_CGU_H' followed
by '#define' of a different macro [-Wheader-guard]
   36 | #ifndef LPC43XX_CGU_H
include/libopencm3/lpc43xx/cgu.h:37: note: 'CGU_LPC43XX_CGU_H' is defined here; did
you mean 'LPC43XX_CGU_H'?
   37 | #define CGU_LPC43XX_CGU_H

Some digging shows this was erroneously changed, presumably by an overzealous search-and-replace, in 770134b.