gdb/gdbserver: add dependencies for distclean-gnulib

gdb/gdbserver: add dependencies for distclean-gnulib

After commit:

  commit 361cb219351d8b7e39e1962fe77f40aa80657b27
  Date:   Tue Oct 6 10:09:06 2020 +0100

      gnulib: Ensure all libraries are used when building gdb/gdbserver

We now get an error when, at the top level of the build tree, we do
'make distclean'.

The reason for this is that the gnulib directory is cleaned before the
gdb directory, cleaning gnulib deletes Makefile.gnulib.inc from the
gnulib build directory, which is currently pulled in by the gdb
Makefile.in using 'include'.

This commit adds a dependency between distclean-gnulib and both
distclean-gdb and distclean-gdbserver.  This means that gdb and
gdbserver will be cleaned before gnulib, as a result the
Makefile.gnulib.inc file should exist when needed.

ChangeLog:

	* Makefile.in: Rebuild.
	* Makefile.def: Make distclean-gnulib depend on distclean-gdb and
	distclean-gdbserver.