cext: make HgRevlogIndex_GetParents private again

cext: make HgRevlogIndex_GetParents private again

The rust's direct-ffi code need to access this function. Now that it
`direct-ffi` is no more. We can make this function `static` again.

This change was suggested by `Yuya Nishihara`.

--- a/mercurial/cext/revlog.c	Fri Mar 06 16:49:46 2020 +0100
+++ b/mercurial/cext/revlog.c	Sun Mar 08 16:40:39 2020 +0100
@@ -208,7 +208,7 @@
  *
  * Returns 0 on success or -1 on failure.
  */
-int HgRevlogIndex_GetParents(PyObject *op, int rev, int *ps)
+static int HgRevlogIndex_GetParents(PyObject *op, int rev, int *ps)
 {
 	int tiprev;
 	if (!op || !HgRevlogIndex_Check(op) || !ps) {

--- a/mercurial/cext/revlog.h	Fri Mar 06 16:49:46 2020 +0100
+++ b/mercurial/cext/revlog.h	Sun Mar 08 16:40:39 2020 +0100
@@ -14,6 +14,4 @@
 
 #define HgRevlogIndex_Check(op) PyObject_TypeCheck(op, &HgRevlogIndex_Type)
 
-int HgRevlogIndex_GetParents(PyObject *op, int rev, int *ps);
-
 #endif /* _HG_REVLOG_H_ */