libpng: use GCC visibility to reduce shared library size

The change introduced the implementation for macro PNG_IMPEXP by using
GCC visibility in defaulting of "hidden" attribute.

Experiment on HTC Passion:
target file; out/target/product/passion/system/lib/libskia.so
(libpng was statically linked to libskia.so)

before: 1117888 bytes
after:  1113780 bytes

Code size reduction = 4108 bytes

Change-Id: I9b718a6a5710268dbb945feea3b64ca3fa393b94
2 files changed