Just make $(HDR) a prereq for $(OBJ)

The result is always correct but it might do redundant builds.
Not really an issue as sbase builds very fast.  The alternative is
to track the header deps manually but this is already incomplete and
unlikely to be kept up to date.

Makefile

@@ -128,9 +128,7 @@ binlib: util.a
 
 bin: $(BIN)
 
-$(OBJ): util.h config.mk
-cat.o fold.o grep.o nl.o sort.o tail.o uniq.o: text.h
-cp.o mv.o rm.o: fs.h
+$(OBJ): $(HDR) config.mk
 
 .o:
 	@echo LD $@