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: 1338a4b7Reported-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

Makefile

@@ -518,6 +518,7 @@ ROM_DIRS_RULES=$(foreach t, all clean, $(addsuffix /$(t), $(ROM_DIRS)))
 $(ROM_DIRS_RULES):
 	$(call quiet-command,$(MAKE) $(SUBDIR_MAKEFLAGS) -C $(dir $@) V="$(V)" TARGET_DIR="$(dir $@)" CFLAGS="$(filter -O% -g%,$(CFLAGS))" $(notdir $@),)
 
+.PHONY: recurse-all recurse-clean recurse-install
 recurse-all: $(addsuffix /all, $(TARGET_DIRS) $(ROM_DIRS))
 recurse-clean: $(addsuffix /clean, $(TARGET_DIRS) $(ROM_DIRS))
 recurse-install: $(addsuffix /install, $(TARGET_DIRS))