meson.build: Don't use fallback dep for libpng unnecessarily

We could have well found the headers and .lib's needed for libpng, so
only use the fallback wrapper for libpng when we couldn't find the
headers and .lib's.
2 jobs from build.fixes in 5 minutes and 3 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #237682
meson-fedora-x86_64

00:01:43

passed #237683
win32
msys2-mingw64

00:05:03