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>
diff --git a/Makefile b/Makefile
index c63de4e..1fcbaed 100644
--- a/Makefile
+++ b/Makefile
@@ -518,6 +518,7 @@
 $(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))