fltk: use libpng version range

[mayeut]

Specify library name and version: fltk/all

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• I've read the contributing guidelines.
• I've used a recent Conan client version close to the currently deployed.
• I've tried at least one configuration locally with the conan-center hook activated.

[ghost]

I detected other pull requests that are modifying fltk/all recipe:

• fltk: Use the mesa-glu Conan package #22958

This message is automatically generated by https://github.com/ericLemanissier/conan-center-conflicting-prs so don't hesitate to report issues/improvements there.

[conan-center-bot]

Conan v1 pipeline ✔️

All green in build 1 (1ee4f06c9aecf843804d463291f9554a0d325443):

• fltk/1.3.8:
  All packages built successfully! (All logs)

• fltk/1.3.9:
  All packages built successfully! (All logs)

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Conan v2 pipeline ✔️

Note: Conan v2 builds are now mandatory. Please read our discussion about it.

All green in build 1 (1ee4f06c9aecf843804d463291f9554a0d325443):

• fltk/1.3.9:
  All packages built successfully! (All logs)

• fltk/1.3.8:
  All packages built successfully! (All logs)