x509_crt: Add initialization to time now

[ThePseudo]

Description

When -Werror is set on the compiler together with -Wuninitialized-const-pointer, we get an error on library/x509_crt.c:2631:37.

This patch fixes this issue.

If MBEDTLS_HAVE_TIME_DATE is defined, then it is initialized. Otherwise, having an initialized structure can be helpful for error checking.

This should also be ported into 4.1 and development branches

PR checklist

Please remove the segment/s on either side of the | symbol as appropriate, and add any relevant link/s to the end of the line.
If the provided content is part of the present PR remove the # symbol.

• changelog not required because: trivial change
• framework PR not required: does not change
• TF-PSA-Crypto development PR not required because: does not change
• TF-PSA-Crypto 1.1 PR not required because: does not change
• mbedtls development PR not required because: trivial change
• mbedtls 4.1 PR not required because: trivial change
• mbedtls 3.6 PR not required because: trivial change
• tests not required because: nothing new was added

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[aduskett]

Anyone want to apply this?