[libc++abi] Remove unused include of <sys/types.h>

[ldionne]

Summary

I ran into an error while trying to build libc++abi for a platform that
doesn't have <sys/types.h>. I couldn't find what <sys/types.h> was used
for in the header, so I think it's fine to remove it.

[ldionne]

Adding @EricWF since he wrote this.

[compnerd]

I agree, as long as declarations are available this should be fine. Even better - this is in the src tree, so we don't even have to worry about users.

Might be useful to just run IWYU over the tree at some point to remove extraneous headers.

[mclow.lists]

In D82810#2123005, @compnerd wrote:

Might be useful to just run IWYU over the tree at some point to remove extraneous headers.

As long as no one deletes includes that may not be used, but are mandated by the standard.

libcxxabi/src/cxa_guard_impl.h

  *    The implementation was designed to allow each implementation to be tested
  *    independent of the C++ runtime or platform support.
  *
  */
 
 #include "__cxxabi_config.h"
 #include "include/atomic_support.h"
 #include <unistd.h>
-#include <sys/types.h>
 #if defined(__has_include)
 # if __has_include(<sys/syscall.h>)
 #   include <sys/syscall.h>
 # endif
 #endif
 
 #include <stdlib.h>
 #include <__threading_support>