build: silence warning about CMakeLists.txt from SPM

Ignore the CMakeLists.txt which is used for the CMake build to support
swift-format.

Co-authored-by: Kyle <kyle201817146@gmail.com>

Author: compnerd

Package.swift

@@ -28,6 +28,9 @@ let package = Package(
                 "CAtomic",
                 .product(name: "cmark-gfm", package: cmarkPackageName),
                 .product(name: "cmark-gfm-extensions", package: cmarkPackageName),
+            ], 
+            exclude: [
+                "CMakeLists.txt"
             ]),
         .testTarget(
             name: "MarkdownTests",