build.zig: fix typo in extension exclusion list

we've been shipping these test files for a while now. oops.

build.zig

@@ -142,8 +142,8 @@ pub fn build(b: *std.Build) !void {
                 ".gz",
                 ".z.0",
                 ".z.9",
-                ".zstd.3",
-                ".zstd.19",
+                ".zst.3",
+                ".zst.19",
                 "rfc1951.txt",
                 "rfc1952.txt",
                 "rfc8478.txt",