[build] Fix cmake config for reflection test without dynamic stdlib

Build SwiftReflectionTest only if swift-reflection-test is available
because it depends on the tool and the tool is not available when no
dynamic stdlib is built

Author: kateinoigakukun

stdlib/private/SwiftReflectionTest/CMakeLists.txt

@@ -1,5 +1,5 @@
 
-if (SWIFT_INCLUDE_TESTS)
+if (SWIFT_INCLUDE_TESTS AND SWIFT_BUILD_DYNAMIC_STDLIB)
   add_swift_target_library(swiftSwiftReflectionTest ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_STDLIB
     SwiftReflectionTest.swift
     SWIFT_COMPILE_FLAGS ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}