Typo in iwyu.cmake (#447)

cmake/iwyu.cmake

@@ -2,6 +2,6 @@ find_program(iwyu_path NAMES include-what-you-use iwyu)
 if(iwyu_path)
   set_property(TARGET ${lib}
     PROPERTY ${iwyu_path} -Xiwyu
-      --mapping_file ${CMAKE_CURRENT_SOURCE_DIR}/iwyu.impl
+      --mapping_file ${CMAKE_CURRENT_SOURCE_DIR}/iwyu.imp
   )
 endif()