[libunwind] Remove unused includes

[whitequark]

Summary

These include directives seem to be carried over from libc++abi. Nothing defined in them seems to be used and they do not break build of libunwind on any compiler/platform I tried. (Unlike #include <algorithm>, which is apparently necessary for static_assert on gcc.)

[rengolin]

This looks like silly left-overs... LGTM, but I'd rather at least someone else would vouch for that, as I'm not building libc++abi that frequently.

[logan]

LGTM. Tested on x86_64 and arm-linux-gnueabihf.

[EricWF]

@whitequark Do you need somebody to commit this for you?

[whitequark]

Missed the LGTM, sorry. Will commit the next time I'm near git-svn.

[whitequark]

r246143

src/DwarfParser.hpp

 #include <stdio.h>
 #include <stdlib.h>
 
-#include <vector>
-
 #include "libunwind.h"
 #include "dwarf2.h"
 

src/Registers.hpp

 #define __REGISTERS_HPP__
 
 #include <stdint.h>
-#include <strings.h>
 #include <string.h>
 
 #include "libunwind.h"

src/libunwind.cpp

 #include <cstdlib> // getenv
 #endif
 #include <new>
-#include <tuple>
-#include <memory>
-#include <vector>
 #include <algorithm>
 
 #include "libunwind_ext.h"