compile error if restrict keyword not supported

[alfredh]

the restrict keyword is part of C99 but if the compiler does not support it,
compilation will fail.

Reference:

https://github.com/baresip/re/blob/main/include/re_fmt.h#L90

There are some compilers that partially supports C99, such as MSVC 2019 version 16.

[alfredh]

other projects are using a macro wrapper:

https://ffmpeg.org/doxygen/1.2/attributes_8h.html#a69c6b995228f305482ad544113a51673

[sreimers]

Looks like we can use __restrict as replacement, made a PR:

#514

https://docs.microsoft.com/en-us/cpp/cpp/extension-restrict?view=msvc-170