constant_time.h: add #ifdef __cplusplus guard

[nbfalcon]

All other headers have a __cplusplus guard, except for constant_time.h**. This allows it to be conveniently included in C++ code.

** as verified using grep -L "#ifdef _cplusplus" mbedtls/.h psa/.h, excepting "config" headers.

Workaround: Users using the older library can simply work around this by

extern "C" {
#include <mbedtls/constant_time.h>
}

Description

Allow constant_time.h to be included in a c++ source file without linker errors.

PR checklist

Please tick as appropriate and edit the reasons (e.g.: "backport: not needed because this is a new feature")

• [X] changelog not required, since this is a trivial change?
• [X] 3.6 backport TODO
• [X] 2.28 backport TODO
• [X] tests not required, since this does not change run-time behaviour. A check for this could be done using a lint?

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