Restore __cplusplus guard absent from oid.h

[HelloOO7]

Description

Since release 4.0.0, mbedtls/oid.h no longer includes an extern "C" guard for when the header is used from C++ code. This makes such C++ code fail to compile with an undefined reference error unless manually wrapped in extern C. I've had this happen in an ESP-IDF project.

This appears to be a regression, as the guard was present up until and including this commit:

mbedtls/include/mbedtls/oid.h

Line 469 in 791fc2e

#ifdef __cplusplus

Since this looked like a rather rudimentary bug to fix, I've created this PR to restore the appropriate preprocessor guards to the file. I apologize if this should have been an issue first, it did not appear necessary to me to create one for such a tiny little thing.

PR checklist

• changelog not required because of the fix being overly trivial to mention
• framework PR - not applicable
• TF-PSA-Crypto development PR - not applicable
• TF-PSA-Crypto 1.1 PR - not applicable
• mbedtls development PR provided
• mbedtls 4.1 PR provided # | not required because:
• mbedtls 3.6 PR not required because this issue did not exist in 3.x
• tests not required because this is a preprocessor guard, not a feature