include zlib.h

libpng no longer include zlib.h itself so now included explicitly

Fixes build error:
pngtocss.c: In function ‘version_info’:
pngtocss.c:100: error: ‘ZLIB_VERSION’ undeclared (first use in this
function)
pngtocss.c:100: error: (Each undeclared identifier is reported only once
pngtocss.c:100: error: for each function it appears in.)
pngtocss.c:100: error: ‘zlib_version’ undeclared (first use in this
function)