Contribs: update libpng to 1.5.13

(cherry picked from commit 34b0ec9a540d974de8e54db85802636372260ea5)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/png/SHA512SUMS

-3edaeb2aa879438f9b71a178fb7bb52fc92589f7544de754d0fafdc5d371eaa19edfe9b771ac9cc0bb674c20108fa2d58b6aa7136f70a588d6b2ef0d1e141324  libpng-1.5.12.tar.bz2
+b31871bbbe647999e2b2a298a69a498bc0779395bf4050574e9a7ffdad717f2ec60e7a42adf308374c8f1dbcc10526ff6e890ab4adb6b249c7c153d8fcd2c462  libpng-1.5.13.tar.bz2

contrib/src/png/rules.mak

 # PNG
-PNG_VERSION := 1.5.12
+PNG_VERSION := 1.5.13
 PNG_URL := $(SF)/libpng/libpng-$(PNG_VERSION).tar.bz2
 
 PKGS += png