Fix compilation warning on MS-Windows in my_png_error.

 src/image.c (fn_png_longjmp) [WINDOWSNT]: Mark the function as
 having the PNG_NORETURN attribute, to avoid compiler warning in
 my_png_error.

src/ChangeLog

+2013-12-21  Eli Zaretskii  <eliz@gnu.org>
+
+	* image.c (fn_png_longjmp) [WINDOWSNT]: Mark the function as
+	having the PNG_NORETURN attribute, to avoid compiler warning in
+	my_png_error.
+
 2013-12-21  YAMAMOTO Mitsuharu  <mituharu@math.s.chiba-u.ac.jp>
 
 	* w32term.h (struct scroll_bar): Remove member `fringe_extended_p'.

src/image.c

@@ -5536,7 +5536,7 @@ DEF_IMGLIB_FN (void, png_read_end, (png_structp, png_infop));
 DEF_IMGLIB_FN (void, png_error, (png_structp, png_const_charp));
 
 #if (PNG_LIBPNG_VER >= 10500)
-DEF_IMGLIB_FN (void, png_longjmp, (png_structp, int));
+DEF_IMGLIB_FN (void, png_longjmp, (png_structp, int)) PNG_NORETURN;
 DEF_IMGLIB_FN (jmp_buf *, png_set_longjmp_fn, (png_structp, png_longjmp_ptr, size_t));
 #endif /* libpng version >= 1.5 */