add extern and comment

git-svn-id: https://svn.r-project.org/R/trunk@87885 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: ripley

src/library/tools/src/text.c

@@ -30,7 +30,8 @@
 LibExtern Rboolean mbcslocale;
 LibExtern int R_MB_CUR_MAX;
 
-size_t Rf_mbrtowc(wchar_t *wc, const char *s, size_t n, mbstate_t *ps);
+// From Defn.h, rmapped from Mbrtowc in src/main/util.c
+extern size_t Rf_mbrtowc(wchar_t *wc, const char *s, size_t n, mbstate_t *ps);
 
 /* .Call, so manages R_alloc stack */
 SEXP