subscriber: fix unused method warnings for Extensions (#1686)

## Motivation

Currently, there are some `pub(crate)` methods on the `Extensions`,
`ExtensionsMut`, and `ExtensionsInner` types that are only used when the
"registry" feature is enabled. However, the _types_ exist when the "std"
feature flag is enabled, so those methods produce unused method
warnings.

## Solution

This commit adds `#[cfg(feature = "registry")]` to those methods, fixing
the warning.

tracing-subscriber/src/registry/extensions.rs

@@ -120,6 +120,7 @@ pub(crate) struct ExtensionsInner {
 
 impl ExtensionsInner {
     /// Create an empty `Extensions`.
+    #[cfg(any(test, feature = "registry"))]
     #[inline]
     #[cfg(any(test, feature = "registry"))]
     pub(crate) fn new() -> ExtensionsInner {