[XRay][compiler-rt] Add missing include to <string>
[XRay][compiler-rt] Add missing include to <string>

Fixes a build break when using clang-3.9.1 (reported upstream,
post-commit review of D30850).

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@298039 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt