Makefile: Fix "make clean" in "unconfigured" source directory

Makefile: Fix "make clean" in "unconfigured" source directory

Recent commit "Makefile: Reuse all's recursion machinery for clean and
install" broke targets clean and distclean in the source directory
before running configure:

    $ make clean
      LD      recurse-clean.mo
    cc: fatal error: no input files
    compilation terminated.
    make: *** [rules.mak:118: recurse-clean.mo] Error 1

Root cause is missing .PHONY.  Fix that.

Fixes: 1338a4b72659ce08eacb9de0205fe16202a22d9c
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>