asyncrt_utils.h: include xlocale on macOS

[barracuda156]

Nothing seems to define HAVE_XLOCALE_H, at least on macOS, but including that header may be required for the build to work.

[barracuda156]

Failure without the fix:

In file included from /opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_www_cpprestsdk/cpprestsdk/work/cpprestsdk-2.10.19/Release/include/cpprest/json.h:18,
                 from /opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_www_cpprestsdk/cpprestsdk/work/cpprestsdk-2.10.19/Release/src/pch/stdafx.h:88:
/opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_www_cpprestsdk/cpprestsdk/work/cpprestsdk-2.10.19/Release/include/cpprest/asyncrt_utils.h: At global scope:
/opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_www_cpprestsdk/cpprestsdk/work/cpprestsdk-2.10.19/Release/include/cpprest/asyncrt_utils.h:317:13: error: 'locale_t' does not name a type
  317 |     typedef locale_t xplat_locale;
      |             ^~~~~~~~
/opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_www_cpprestsdk/cpprestsdk/work/cpprestsdk-2.10.19/Release/include/cpprest/asyncrt_utils.h:320:24: error: 'xplat_locale' does not name a type
  320 |     static _ASYNCRTIMP xplat_locale __cdecl c_locale();
      |                        ^~~~~~~~~~~~
/opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_www_cpprestsdk/cpprestsdk/work/cpprestsdk-2.10.19/Release/include/cpprest/asyncrt_utils.h:327:5: error: 'locale_t' does not name a type
  327 |     locale_t m_prevLocale;
      |     ^~~~~~~~

[ryandesign]

The error this fixes was previously reported at https://github.com/microsoft/cpprestsdk/issues/1381#issuecomment-659977838