Fix gdb.trace/ftrace-lock.c compilation

Fixes, on F23:

 .../src/gdb/testsuite/gdb.trace/ftrace-lock.c: In function 'gdb_agent_gdb_collect':
 .../src/gdb/testsuite/gdb.trace/ftrace-lock.c:50:3: warning: implicit declaration of function 'sleep' [-Wimplicit-function-declaration]
    sleep (1);
    ^

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

	* gdb.trace/ftrace-lock.c: Include <unistd.h>.