gh-107211: Fix select extension build on Solaris

Export the internal _Py_open() function for Solaris: the select
extension uses it in its newDevPollObject() function.

Author: vstinner

Include/internal/pycore_fileutils.h

@@ -109,7 +109,8 @@ PyAPI_FUNC(int) _Py_stat(
     PyObject *path,
     struct stat *status);
 
-extern int _Py_open(
+// Export for 'select' shared extension (Solaris newDevPollObject() uses it)
+PyAPI_FUNC(int) _Py_open(
     const char *pathname,
     int flags);