FindBoost: pop policy stack before returning

CMake would give a fatal error if the policy stack was cleaned up
upon exiting the module.

Modules/FindBoost.cmake

@@ -244,6 +244,8 @@ if (NOT Boost_NO_BOOST_CMAKE)
       message("Found Boost components:")
       message("   ${Boost_FIND_COMPONENTS}")
     endif()
+    # Restore project's policies
+    cmake_policy(POP)
     return()
   endif()
 endif()