Using `-Diconv-impl=libiconv` creates build error

[silver-signal]

Reproduction:

zig build -Diconv-impl=libiconv

Looks like just a typo when linking with libiconv:

libxml2/build.zig

Lines 214 to 221 in eddd40e

	.libiconv => {
	if (b.lazyDependency("libiconv", .{
	.target = target,
	.optimize = optimize,
	})) |libiconv_dependency| {
	xml_lib.root_module.linkLibrary(libiconv_dependency.artifact("z"));
	}
	},

I can submit a fix in the next week or two.