bdiff: avoid pointer arithmetic on void*

MSVC 2008 complains:

  mercurial/cext/bdiff.c(106) : error C2036: 'void *' : unknown size
  mercurial/cext/bdiff.c(107) : error C2036: 'void *' : unknown size

Maybe it's a gcc extension?

https://stackoverflow.com/questions/37460579/error-c2036-void-unknown-size

mercurial/cext/bdiff.c

@@ -103,8 +103,8 b' static PyObject *bdiff(PyObject *self, P'
 	}
 	/* we can almost add: if (li == lmax) lcommon = li; */
 
-	an = bdiff_splitlines(ba.buf + lcommon, la - lcommon, &al);
-	bn = bdiff_splitlines(bb.buf + lcommon, lb - lcommon, &bl);
+	an = bdiff_splitlines((char *)ba.buf + lcommon, la - lcommon, &al);
+	bn = bdiff_splitlines((char *)bb.buf + lcommon, lb - lcommon, &bl);
 	if (!al || !bl) {
 		PyErr_NoMemory();
 		goto cleanup;