fix(auth): correct sign-compare warning

fix(auth): correct sign-compare warning

Correct the warning due to comparison between signed and
unsigned variable.

drivers/auth/mbedtls/mbedtls_x509_parser.c: In function 'get_ext':
drivers/auth/mbedtls/mbedtls_x509_parser.c:120:30:
	error: comparison of integer expressions of different
	signedness: 'int' and 'size_t' {aka 'unsigned int'}
	[-Werror=sign-compare]
120 | if ((oid_len == strlen(oid_str)) && !strcmp(oid, oid_str)) {
    |              ^~

Change-Id: Ic12527f5f92a34e925bee3047c168eacf5e99d8a
Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@st.com>