libpng: upgrade 1.6.47 -> 1.6.48

libpng: upgrade 1.6.47 -> 1.6.48

Changelog:
============
-  Fixed the floating-point version of the mDCv setter 'png_set_mDCv'.
-  Added #error directives to discourage the inclusion of private
   libpng implementation header files in PNG-supporting applications.
-  Added the CMake build option 'PNG_LIBCONF_HEADER', to be used as an
   alternative to 'DFA_XTRA'.
-  Removed the Travis CI configuration files

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>