Include header for enum target_stop_reason

Include header for enum target_stop_reason

Building in C++ mode errors with:

 ~~~
 g++ -fpermissive (...) /home/pedro/gdb/mygit/src/gdb/gdbserver/../nat/x86-linux.c
 In file included from /home/pedro/gdb/mygit/src/gdb/gdbserver/../nat/x86-linux.h:23:0,
		  from /home/pedro/gdb/mygit/src/gdb/gdbserver/../nat/x86-linux.c:21:
 /home/pedro/gdb/mygit/src/gdb/gdbserver/../nat/linux-nat.h:74:13: error: use of enum ‘target_stop_reason’ without previous declaration
  extern enum target_stop_reason lwp_stop_reason (struct lwp_info *lwp);
	      ^
 /home/pedro/gdb/mygit/src/gdb/gdbserver/../nat/linux-nat.h:74:70: error: invalid type in declaration before ‘;’ token
  extern enum target_stop_reason lwp_stop_reason (struct lwp_info *lwp);
								       ^
 ~~~

gdb/ChangeLog:
2015-05-15  Pedro Alves  <palves@redhat.com>

	* nat/linux-nat.h: Include "target/waitstatus.h".