fix(pdf): update linker flags for PNG

[kojix2]

Hello.
On my macOS system, it fails to find Homebrew's libpng.

❯ shards build --release
Dependencies are satisfied
Building: markterm
Building: markmark
Building: markpdf
Error target markpdf failed to compile:
ld: warning: ignoring duplicate libraries: '-llitepdf', '-lz'
ld: library 'png' not found
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I had to make this change.
I’m not sure if this fix works on all platforms, so I’m just offering it as a suggestion.
Thank you.

[ralsina]

Should be fine.