Fix gdb.arch/aarch64-dbreg-contents.exp build failures

I ran into the following failures when running tests under QEMU:

--

gdb compile failed, binutils-gdb/gdb/testsuite/gdb.arch/aarch64-dbreg-contents.c: In function 'set_watchpoint':
binutils-gdb/gdb/testsuite/gdb.arch/aarch64-dbreg-contents.c:41:29: error: storage size of 'dreg_state' isn't known
   struct user_hwdebug_state dreg_state;
                             ^~~~~~~~~~
In file included from /usr/include/aarch64-linux-gnu/bits/types/struct_iovec.h:23:0,
                 from /usr/include/aarch64-linux-gnu/sys/uio.h:23,
                 from binutils-gdb/gdb/testsuite/gdb.arch/aarch64-dbreg-contents.c:17:
binutils-gdb/gdb/testsuite/gdb.arch/aarch64-dbreg-contents.c:69:18: error: invalid use of undefined type 'struct user_hwdebug_state'
   iov.iov_len = (offsetof (struct user_hwdebug_state, dbg_regs)
                  ^
binutils-gdb/gdb/testsuite/gdb.arch/aarch64-dbreg-contents.c:74:5: warning: implicit declaration of function 'error'; did you mean 'errno'? [-Wimplicit-function-declaration]
     error (1, errno, "PTRACE_SETREGSET: NT_ARM_HW_WATCH");
     ^~~~~
     errno
binutils-gdb/gdb/testsuite/gdb.arch/aarch64-dbreg-contents.c: In function 'main':
binutils-gdb/gdb/testsuite/gdb.arch/aarch64-dbreg-contents.c:87:3: warning: implicit declaration of function 'atexit'; did you mean '_Exit'? [-Wimplicit-function-declaration]
   atexit (cleanup);
   ^~~~~~
   _Exit
binutils-gdb/gdb/testsuite/gdb.arch/aarch64-dbreg-contents.c:89:11: warning: implicit declaration of function 'fork' [-Wimplicit-function-declaration]
   child = fork ();

           ^~~~

--

The following patch fixes those by adding the necessary include files.

With that said, the test doesn't pass at present. I'll have to investigate it
a bit more.

gdb/testsuite/ChangeLog:

2020-02-28  Luis Machado  <luis.machado@linaro.org>

	* gdb.arch/aarch64-dbreg-contents.c: Include stdlib.h, unistd,
	asm/ptrace.h and error.h.