fix(st): include utils.h to solve compilation error

fix(st): include utils.h to solve compilation error

If compiling with STM32MP13 with DECRYPTION_SUPPORT != none, there is
a compilation error:
plat/st/common/stm32mp_crypto_lib.c:
 In function 'plat_get_enc_key_info':
plat/st/common/stm32mp_crypto_lib.c:532:25:
 error: implicit declaration of function 'zeromem'
 [-Werror=implicit-function-declaration]
  532 |                         zeromem(key, *key_len);
      |                         ^~~~~~~

Adding #include <lib/utils.h> solves the error.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I0a20c5632f0379612149333e69875369d4cfca15