[compiler-rt][lsan] Fix compiler error due to attribute (windows)

Summary:
Window compiler is stricter for attributes location. This patch fixes a compilation error.
```
D:\src\llvm\llvm\projects\compiler-rt\lib\lsan\lsan_thread.cc(39): error C2144: syntax error: 'int' should be preceded by ';'
```

Reviewers: rnk, majnemer

Subscribers: majnemer, llvm-commits, chrisha, dberris

Differential Revision: https://reviews.llvm.org/D24810

llvm-svn: 282254