ExtensionAttribute must be public to prevent compiler errors

The compiler ignores that it's internal and generates an error when
compiling other DLLs. Making it public solves the error but there
are still warnings (which can be ignored).

src/ExtensionAttribute.cs

@@ -26,6 +26,6 @@ SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 namespace System.Runtime.CompilerServices {
 	[AttributeUsage(AttributeTargets.Assembly | AttributeTargets.Class | AttributeTargets.Method)]
-	sealed class ExtensionAttribute : Attribute {
+	public sealed class ExtensionAttribute : Attribute {
 	}
 }