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.