fix(st-crypto): set get_plain_pk_from_asn1() static

The issue is triggered when enabling -Wmissing-prototypes warning:
plat/st/common/stm32mp_crypto_lib.c:81:5: warning:
 no previous prototype for 'get_plain_pk_from_asn1'
 [-Wmissing-prototypes]
   81 | int get_plain_pk_from_asn1(void *pk_ptr, unsigned int pk_len,
      |     ^~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ia36bbaba6e187ab47c203ddf0d7d017b210545cf