btrfs-progs: build: add rule for cleaning build dependencies

The header dependency rules generated as .o.d files are sometimes stale
and fail the build. Add a rule to clean them if needed, otherwise
they're also cleaned by 'make clean'.

Signed-off-by: David Sterba <dsterba@suse.com>

Makefile

@@ -10,6 +10,7 @@
 #               to default locationh (/usr/local)
 #   clean       clean built binaries (not the documentation)
 #   clean-all   clean as above, clean docs and generated files
+#   clean-dep   clean header dependency files (*.o.d)
 #
 # All-in-one binary (busybox style):
 #   btrfs.box         single binary with functionality of mkfs.btrfs, btrfs-image,
@@ -816,6 +817,10 @@ clean-gen:
 		config.log config.h config.h.in~ aclocal.m4 \
 		configure autom4te.cache/ config/
 
+clean-dep:
+	@echo "Cleaning dependency files"
+	$(Q)$(RM) -f -- *.o.d */*.o.d */*/*.o.d
+
 $(CLEANDIRS):
 	@echo "Cleaning $(patsubst clean-%,%,$@)"
 	$(Q)$(MAKE) $(MAKEOPTS) -C $(patsubst clean-%,%,$@) clean