gn build: Don't assume valgrind.h exists on Linux

It didn't on my machine, so defaulting it to off seems better.

Differential revision: https://reviews.llvm.org/D56727

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351202 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed