Add MSVC pragma for UTF-8 source code

Not sure we have any right now; motivated by this:
https://bugzilla.gnome.org/show_bug.cgi?id=747772

src/hb-private.hh

@@ -132,6 +132,7 @@ static int errno = 0; /* Use something better? */
 #  endif
 #  if defined(_MSC_VER) && _MSC_VER < 1900
 #    define snprintf _snprintf
+#    pragma execution_character_set("utf-8")
 #  endif
 #endif