Install _CryptoExtras target when building with CMake

--- a/Sources/_CryptoExtras/CMakeLists.txt
+++ b/Sources/_CryptoExtras/CMakeLists.txt
@@ -42,4 +42,5 @@ target_link_options(_CryptoExtras PRIVATE
 set_target_properties(_CryptoExtras PROPERTIES
   INTERFACE_INCLUDE_DIRECTORIES ${CMAKE_Swift_MODULE_DIRECTORY})

+_install_target(_CryptoExtras)
 set_property(GLOBAL APPEND PROPERTY SWIFT_CRYPTO_EXPORTS _CryptoExtras)