patch 8.1.0615: get_tv function names are not consistent

Problem:    Get_tv function names are not consistent.
Solution:   Rename to tv_get.
diff --git a/src/os_win32.c b/src/os_win32.c
index f610d21..c9f7d1c 100644
--- a/src/os_win32.c
+++ b/src/os_win32.c
@@ -5355,7 +5355,7 @@
 	    {
 		typval_T *item = &dict_lookup(hi)->di_tv;
 		WCHAR   *wkey = enc_to_utf16((char_u *)hi->hi_key, NULL);
-		WCHAR   *wval = enc_to_utf16(get_tv_string(item), NULL);
+		WCHAR   *wval = enc_to_utf16(tv_get_string(item), NULL);
 		--todo;
 		if (wkey != NULL && wval != NULL)
 		{