IWYU: Add mapping for bits/chrono.h to chrono

Utilities/IWYU/mapping.imp

@@ -22,6 +22,7 @@
 
   # HACK: check whether this can be removed with next iwyu release.
   { include: [ "<bits/cxxabi_forced.h>", private, "<ctime>", public ] },
+  { include: [ "<bits/chrono.h>", private, "<chrono>", public ] },
   { include: [ "<bits/exception.h>", private, "<exception>", public ] },
   { include: [ "<bits/shared_ptr.h>", private, "<memory>", public ] },
   { include: [ "<bits/std_function.h>", private, "<functional>", public ] },