Rename "include/CgRPC.h" to "cgrpc.h".

This is actually only a symlink to "shim/cgrpc.h", so I figured it would make sense to have both names in a consistent case.

Sources/CgRPC/include/module.modulemap

@@ -1,4 +1,4 @@
 module CgRPC {
-    header "CgRPC.h"
+    header "cgrpc.h"
     export *
 }