Contribs: update libpng to 1.5.14

(cherry picked from commit aad495b365fdbec856537bd06650a85283c7b4da)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/png/SHA512SUMS

-b31871bbbe647999e2b2a298a69a498bc0779395bf4050574e9a7ffdad717f2ec60e7a42adf308374c8f1dbcc10526ff6e890ab4adb6b249c7c153d8fcd2c462  libpng-1.5.13.tar.bz2
+5fba9227e991f7cba6b45d687bd48984d005c072f6de05cb2b6bd9cab7b816aceae85986fcf0b33ae6110dc71852f5452ef3914ed7d8a5b8f54ede3f2d8448e4  libpng-1.5.14.tar.bz2

contrib/src/png/rules.mak

 # PNG
-PNG_VERSION := 1.5.13
+PNG_VERSION := 1.5.14
 PNG_URL := $(SF)/libpng/libpng-$(PNG_VERSION).tar.bz2
 
 PKGS += png