dependencies/zlib: Fix header detection
dependencies/zlib: Fix header detection

has_header returns a tuple of (found: bool, cached: bool), so `if
has_header` will always return true because the tuple is non-empty. We
need to check if the found value is true or not.

mesonbuild/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,QuLogic/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,QuLogic/meson,jpakkane/meson,jpakkane/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,jpakkane/meson,pexip/meson,QuLogic/meson,jpakkane/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,QuLogic/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson