lib: mbedtls: do not use std=c99 on Windows
lib: mbedtls: do not use std=c99 on Windows

Visual Studio does not recognize `-std=c99`. For this reason, it emits
the following error while compiling:

    cl : Command line warning D9002 : ignoring unknown option '-std=c99'

Note: this particular line was introduced by cdf3a396e6. The upstream
project does not have the corresponding line, so we need to fix it here.

Signed-off-by: Fujimoto Seiji <ef879192bb1e41d519c82cf76440dc8e9a530806@ceptord.net>

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