Clarify GDBSERVER use in linux-waitpid.c

This commit makes linux-waitpid.c include common-defs.h.  GDB's
inclusion of defs.h is removed, but gdbserver's inclusion of
server.h remains to support some gdbserver-specific debug code
that cannot presently be merged.  A new FIXME documents this.

gdb/ChangeLog:

	* nat/linux-waitpid.c: Include common-defs.h.
	[GDBSERVER]: Add FIXME comment.
	[!GDBSERVER]: Don't include defs.h or signal.h.
	(linux_debug) [!GDBSERVER]: Remove empty block.