Enable CA1018 and fix noncompliance

"Mark attributes with AttributeUsageAttribute"

Common.ruleset

@@ -32,9 +32,6 @@
 		<!-- Mark assemblies with assembly version -->
 		<Rule Id="CA1016" Action="Hidden" />
 
-		<!-- Mark attributes with AttributeUsageAttribute -->
-		<Rule Id="CA1018" Action="Hidden" />
-
 		<!-- Enum Storage should be Int32 -->
 		<Rule Id="CA1028" Action="Hidden" />
 

src/BizHawk.Client.Common/Api/ExternalToolAttributes.cs

@@ -118,6 +118,7 @@ namespace BizHawk.Client.Common
 		}
 	}
 
+	[AttributeUsage(AttributeTargets.Class)]
 	public sealed class ExternalToolEmbeddedIconAttribute : Attribute
 	{
 		/// <remarks>The full path, including the assembly name.</remarks>