Fix excluded required source file in cmark-gfm

Fixes an excluded source file in `cmark-gfm` that is required by [utf8.c](https://github.com/apple/swift-cmark/blob/gfm/src/utf8.c#L237).

Author: luispadron

Package.swift

@@ -53,7 +53,6 @@ let package = Package(
             "config.h.in",
             "CMakeLists.txt",
             "cmark-gfm_version.h.in",
-            "case_fold_switch.inc",
             "entities.inc",
           ],
           cSettings: cSettings