meson: Fix libpng fallback dependency variable
The libpng subproject dependency variable name was wrong. It's libpng_dep:
https://github.com/mesonbuild/libpng/blob/1.6.37/meson.build#L110
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The libpng subproject dependency variable name was wrong. It's libpng_dep:
https://github.com/mesonbuild/libpng/blob/1.6.37/meson.build#L110