LibGC: Make GC::Root bool operator explicit

Not having this led to a sneaky bug where, given a Variant like so:

  Variant<double, GC::Root<T>>

An incorrectly-written visit() branch for the Root would just cause it
to be cast to a double and call that branch instead.

With the cast made explicit, we get a compiler error, which is far more
useful.

Co-authored-by: Jelle Raaijmakers <jelle@ladybird.org>

Libraries/LibGC/Root.h

@@ -105,7 +105,7 @@ public:
     {
         return !cell();
     }
-    operator bool() const
+    explicit operator bool() const
     {
         return cell();
     }