Update CMakeLists.txt with missing URLTemplate_Expression.swift (#1262)

#1198 added most of
the new files, but missed `URLTemplate_Expression.swift`, add it.

Author: bnbarham

Sources/FoundationEssentials/URL/CMakeLists.txt

@@ -21,6 +21,7 @@ target_sources(FoundationEssentials PRIVATE
     URLComponents.swift
     URLComponents_ObjC.swift
     URLParser.swift
+    URLTemplate_Expression.swift
     URLTemplate_PercentEncoding.swift
     URLTemplate_Substitution.swift
     URLTemplate_Value.swift