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

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 for build.fixes in 4 minutes and 59 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #237667
meson-fedora-x86_64

00:01:33

passed #237668
win32
msys2-mingw64

00:04:59