Add (unused) libflex modulemap file

For my own use case, I dynamically link FLEX, so I need to build Reflex using libflex.tbd as well as this modulemap file after copying it into the Headers folder.

module.modulemap

@@ -0,0 +1,8 @@
+module FLEX {
+  umbrella header "FLEX.h"
+
+  link "flex"
+
+  export *
+  module * { export * }
+}