[libunwind] Move errno.h and signal.h includes under the block where they're needed (#78054)

Commit fc1c478 added includes of <signal.h> and <errno.h> to
UnwindCursor.hpp. The library previously built on platforms where these
headers are not provided. These headers should be included only in the
case where they are actually needed, i.e. on Linux.

Author: ldionne

libunwind/src/UnwindCursor.hpp

@@ -12,8 +12,6 @@
 #define __UNWINDCURSOR_HPP__
 
 #include "cet_unwind.h"
-#include <errno.h>
-#include <signal.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -35,6 +33,8 @@
 #if defined(_LIBUNWIND_TARGET_LINUX) &&                                        \
     (defined(_LIBUNWIND_TARGET_AARCH64) || defined(_LIBUNWIND_TARGET_RISCV) || \
      defined(_LIBUNWIND_TARGET_S390X))
+#include <errno.h>
+#include <signal.h>
 #include <sys/syscall.h>
 #include <sys/uio.h>
 #include <unistd.h>