Annotate mbstowcs_nonfatal() with restrict

The arguments to our mbrtowc(3) wrapper should not alias, since they
also must not for mbrtowc(3).

Might help some compilers optimizing the code.

RichString.c

@@ -64,7 +64,7 @@ void RichString_rewind(RichString* this, int count) {
 
 #ifdef HAVE_LIBNCURSESW
 
-static size_t mbstowcs_nonfatal(wchar_t* dest, const char* src, size_t n) {
+static size_t mbstowcs_nonfatal(wchar_t* restrict dest, const char* restrict src, size_t n) {
    size_t written = 0;
    mbstate_t ps = { 0 };
    bool broken = false;