cpu/stm32: fix include header guards.

cpu/stm32f0/include/cpu_conf.h

cpu/stm32f0/include/stm32f051x8.h

cpu/stm32f0/include/stm32f072xb.h

cpu/stm32f0/include/stm32f091xc.h

cpu/stm32f1/include/stm32f103xb.h

cpu/stm32f1/include/stm32f103xe.h

cpu/stm32f3/include/cpu_conf.h

cpu/stm32f3/include/stm32f303xc.h

cpu/stm32f3/include/stm32f303xe.h

cpu/stm32f3/include/stm32f334x8.h

cpu/stm32f4/include/cpu_conf.h

cpu/stm32f4/include/stm32f401xe.h

cpu/stm32f4/include/stm32f407xx.h

cpu/stm32f4/include/stm32f415xx.h

cpu/stm32l1/include/stm32l1xx.h