gh-106320: Fix test_peg_generator: _Py_UniversalNewlineFgetsWithSize() (#108609)

Fix test_peg_generator by exporting the
_Py_UniversalNewlineFgetsWithSize() function.

Author: vstinner

Include/internal/pycore_fileutils.h

@@ -315,7 +315,8 @@ extern HRESULT PathCchSkipRoot(const wchar_t *pszPath, const wchar_t **ppszRootE
 // Export for 'select' shared extension (Argument Clinic code)
 PyAPI_FUNC(int) _PyLong_FileDescriptor_Converter(PyObject *, void *);
 
-extern char* _Py_UniversalNewlineFgetsWithSize(char *, int, FILE*, PyObject *, size_t*);
+// Export for test_peg_generator
+PyAPI_FUNC(char*) _Py_UniversalNewlineFgetsWithSize(char *, int, FILE*, PyObject *, size_t*);
 
 #ifdef __cplusplus
 }