Remove <linux/unistd.h> include for compatibility with non-glibc

On my glibc (Debian Jessie amd64 if anybody cares) system,
<linux/unistd.h> is a strict subset of <sys/sycall.h>, which the file is
already including. musl libc doesn't provide this file, and with this
change all the C++ tests pass with musl libc.

This came up in bazelbuild/bazel#1492.

src/core/lib/support/log_linux.c

@@ -47,7 +47,6 @@
 #include <grpc/support/log.h>
 #include <grpc/support/string_util.h>
 #include <grpc/support/time.h>
-#include <linux/unistd.h>
 #include <stdarg.h>
 #include <stdio.h>
 #include <string.h>