gdb.threads/clone-thread_db.c: Add missing includes and fix pthread_join call

gdb.threads/clone-thread_db.c: Add missing includes and fix pthread_join call

This fixes:

> gdb compile failed, /gdb/testsuite/gdb.threads/clone-thread_db.c: In function 'main':
> /gdb/testsuite/gdb.threads/clone-thread_db.c:67:3: warning: implicit declaration of function 'alarm' [-Wimplicit-function-declaration]
>    alarm (300);
>    ^
> /gdb/testsuite/gdb.threads/clone-thread_db.c:69:3: warning: implicit declaration of function 'pthread_create' [-Wimplicit-function-declaration]
>    pthread_create (&child, NULL, thread_fn, NULL);
>    ^
> /gdb/testsuite/gdb.threads/clone-thread_db.c:70:3: warning: implicit declaration of function 'pthread_join' [-Wimplicit-function-declaration]
>    pthread_join (child);
>    ^

And then adding the missing headers revealed the pthread_join call was
incorrect.  This probably fixes the crash we see on ppc64be, e.g., at

 https://sourceware.org/ml/gdb-testers/2015-q1/msg04415.html

the logs there show:

 ...
 Program received signal SIGSEGV, Segmentation fault.
 [Switching to Thread 0x3fffb7ff54a0 (LWP 9275)]
 0x00003fffb7f3ce74 in .pthread_join () from /lib64/libpthread.so.0
 (gdb) FAIL: gdb.threads/clone-thread_db.exp: continue to end
 ...

Tested on x86_64 Fedora 20.

gdb/testsuite/
2015-03-04  Pedro Alves  <palves@redhat.com>

	* gdb.threads/clone-thread_db.c: Include unistd.h and pthread.h.
	(main): Pass missing retval argument to pthread_join call.