[libsanitizer] Drive-by fix for -Wempty-body in sanitizer_common_syscalls.inc
This makes the file consistently use { } around the if statements containing the PRE_/POST_ macros.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@187797 91177308-0d34-0410-b5e6-96231b3b80d8