[scudo] Do not compile timing.cpp if LLVM_LIBC_INCLUDE_SCUDO=on

Temporary hack until LLVM libc supports inttypes.h print format macros

timing.h uses the PRId64 macro which is not included in llvm libc yet

Bug: https://github.com/llvm/llvm-project/issues/63317

Reviewed By: michaelrj, thesamesam, Chia-hungDuan

Differential Revision: https://reviews.llvm.org/D152979