std.Build.Step.Compile: change root_module field type to *Module

This commit changes the `root_module` field of `std.Build.Step.Compile`
to be a `*Module` rather than a `Module`. This is a breaking change, but
an incredibly minor one (the full potential extent of the breakage can
be seen in the modified standalone test).

This change will be necessary for an upcoming improvement, so it was
convenient to make it here.

lib/std/Build/Module.zig

@@ -430,7 +430,7 @@ pub const DependencyIterator = struct {
                     if (!it.chase_dyn_libs and compile.isDynamicLibrary()) continue;
 
                     it.set.put(it.allocator, .{
-                        .module = &compile.root_module,
+                        .module = compile.root_module,
                         .compile = compile,
                     }, "root") catch @panic("OOM");
                 },