[libc] Fix mismatch in exception decl

[jeffbailey]

Summary

When running the libc tests under vscode, I got an error about a
mismatches exception declaration for strerror. Since string.h
seems to be getting included transitively anyway, just include
it and rely on its definition.

[michaelrj]

LGTM. If I remember correctly, that extern declaration is a leftover from before we had a version of strerror. Make sure to test both in fullbuild and overlay modes before landing.

libc/test/ErrnoSetterMatcher.h

 //===-- ErrnoSetterMatcher.h ------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIBC_TEST_ERRNOSETTERMATCHER_H
 #define LLVM_LIBC_TEST_ERRNOSETTERMATCHER_H
 
 #include "test/UnitTest/Test.h"
 
 #include <errno.h>
+#include <string.h>
 
 namespace __llvm_libc {
 namespace testing {
 
 namespace internal {
 
-extern "C" char *strerror(int);
-
 template <typename T> class ErrnoSetterMatcher : public Matcher<T> {
   T ExpectedReturn;
   T ActualReturn;
   int ExpectedErrno;
   int ActualErrno;
 
 public:
   ErrnoSetterMatcher(T ExpectedReturn, int ExpectedErrno)