Remove unnecessary inclusion of sys/syscall.h in base/logging.cc

The inclusion of sys/syscall.h was introduced in crrev.com/4916 to
obtain the current thread ID via `syscall(__NR_gettid)`. The code was
later changed in crrev.com/118152 to use
`base::PlatformThread::CurrentId()` instead. The inclusion of
sys/syscall.h is no longer needed and thus removed by this CL.

BUG=None

Review URL: https://codereview.chromium.org/1812733002

Cr-Commit-Position: refs/heads/master@{#381735}

base/logging.cc

@@ -30,8 +30,6 @@ typedef HANDLE MutexHandle;
 #elif defined(OS_POSIX)
 #if defined(OS_NACL)
 #include <sys/time.h>  // timespec doesn't seem to be in <time.h>
-#else
-#include <sys/syscall.h>
 #endif
 #include <time.h>
 #endif