fix(st): add missing string.h include

fix(st): add missing string.h include

Since patch on libc refactoring, there is a compilation error with
STM32MP_USB_PROGRAMMER=1:
plat/st/common/stm32cubeprogrammer_usb.c:81:35: error:
 implicit declaration of function 'strnlen'
 [-Werror=implicit-function-declaration]
      length += strnlen((char *)&dfu->buffer[GET_PHASE_LEN],

The string.h header file should be included.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I1fbb2d9714cbc0d0640cb5e3c5ae8201dbfbe14e