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

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.

Differential Revision: https://reviews.llvm.org/D82810

libcxxabi/src/cxa_guard_impl.h

@@ -40,7 +40,6 @@
 #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>