Remove one GDBSERVER use from linux-waitpid.c

This commit makes nat/linux-waitpid.c include common-defs.h rather
than defs.h or server.h.  Some debug code previously only built for
gdbserver is now also built for GDB, and a use of GDBSERVER has had
to remain in order to select the variable used to determine whether
debug printing should be enabled.

gdb/ChangeLog:

	* linux-nat.c (debug_linux_nat): Make nonstatic.
	* nat/linux-waitpid.c: Include common-defs.h.
	Don't include defs.h, server.h or signal.h.
	(debug_linux_waitpid): New macro and extern.
	(linux_debug): Use debug_linux_waitpid and debug_vprintf.