Add missing PyAPI_FUNC macro's to the public functions as other .c files do

in hopes that this fixes the windows extension module link error for modules
trying to call _Py_strhex functions.  issue9951.

Python/pystrhex.c

@@ -47,14 +47,14 @@ static PyObject *_Py_strhex_impl(const char* argbuf, const Py_ssize_t arglen,
     return retval;
 }
 
-PyObject *_Py_strhex(const char* argbuf, const Py_ssize_t arglen)
+PyAPI_FUNC(PyObject *) _Py_strhex(const char* argbuf, const Py_ssize_t arglen)
 {
     return _Py_strhex_impl(argbuf, arglen, 0);
 }
 
 /* Same as above but returns a bytes() instead of str() to avoid the
  * need to decode the str() when bytes are needed. */
-PyObject *_Py_strhex_bytes(const char* argbuf, const Py_ssize_t arglen)
+PyAPI_FUNC(PyObject *) _Py_strhex_bytes(const char* argbuf, const Py_ssize_t arglen)
 {
     return _Py_strhex_impl(argbuf, arglen, 1);
 }