Fix gdb.threads/watchpoint-fork*.c compilation

Fix gdb.threads/watchpoint-fork*.c compilation

This testcase currently fails to compile on Fedora 23:

 .../src/gdb/testsuite/gdb.threads/watchpoint-fork-mt.c: In function 'start':
 .../src/gdb/testsuite/gdb.threads/watchpoint-fork-mt.c:70:11: warning: implicit declaration of function 'pthread_yield' [-Wimplicit-function-declaration]
	i = pthread_yield ();
	    ^
 .../src/gdb/testsuite/gdb.threads/watchpoint-fork-child.c: In function 'forkoff':
 .../src/gdb/testsuite/gdb.threads/watchpoint-fork-child.c:114:8: warning: implicit declaration of function 'pthread_yield' [-Wimplicit-function-declaratio
 n]
     i = pthread_yield ();
	 ^
 /tmp/ccUkNIsI.o: In function `start':
 .../src/gdb/testsuite/gdb.threads/watchpoint-fork-mt.c:70: undefined reference to `pthread_yield'
 (...)
 collect2: error: ld returned 1 exit status

 UNSUPPORTED: gdb.threads/watchpoint-fork.exp: child: multithreaded: Couldn't compile watchpoint-fork-child.c: unrecognized error
 UNTESTED: gdb.threads/watchpoint-fork.exp: child: multithreaded: watchpoint-fork.exp
 testcase .../src/gdb/testsuite/gdb.threads/watchpoint-fork.exp completed i

The glibc manual says, on _GNU_SOURCE:

 "You should define these macros by using ‘#define’ preprocessor
 directives at the top of your source code files. These directives must
 come before any #include of a system header file."

I instead put it in the header all the .c files of the testcase must
include anyway.

gdb/testsuite/ChangeLog:
2016-03-01  Pedro Alves  <palves@redhat.com>

	* gdb.threads/watchpoint-fork-child.c: Include "watchpoint-fork.h"
	before anything else.
	* gdb.threads/watchpoint-fork-mt.c: Likewise.  Don't define
	_GNU_SOURCE here.
	* gdb.threads/watchpoint-fork-st.c: Include "watchpoint-fork.h"
	before anything else.
	* gdb.threads/watchpoint-fork.h: Define _GNU_SOURCE.