Add missing endif()

Author: MaxDesiatov

stdlib/cmake/modules/SwiftSource.cmake

@@ -286,6 +286,7 @@ function(_add_target_variant_swift_compile_flags
 
   if(SWIFT_STDLIB_HAS_STDIN)
     list(APPEND result "-D" "SWIFT_STDLIB_HAS_STDIN")
+  endif()
 
   if(SWIFT_STDLIB_SINGLE_THREADED_RUNTIME)
     list(APPEND result "-DSWIFT_STDLIB_SINGLE_THREADED_RUNTIME")