build: cmark-gfm.h is not an umbrella header

This header does not cover all headers. Furthermore, it forces the
subsuming of `mutex.h` which is a private header. Fixing the module
definition to repair the macOS build.

Author: compnerd

src/include/module.modulemap

@@ -1,5 +1,5 @@
 module cmark_gfm {
-    umbrella header "cmark-gfm.h"
+    header "cmark-gfm.h"
     header "cmark-gfm-extension_api.h"
     header "buffer.h"
     header "chunk.h"