Protect another R_NO_REMAP with preceding #ifndef

[eddelbuettel]

I noticed a second #define R_NO_REMAP in the (very much optional) file Benchmark/Timer.h so adding a now-required preceding #ifndef / #endif around it as R 4.5.0 now defines R_NO_REMAP for C++ -- just like we have done since 2009 (!!).
There was an earlier PR #1296 where I protected the main #define, but missed this.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[Enchufa2]

LGTM!