rules.mk: Fix parallel build for 'make install'

install-bins was missing a dependency on the binaries themselves.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/rules.mk b/rules.mk
index 758e9f2..fb6c000 100644
--- a/rules.mk
+++ b/rules.mk
@@ -19,7 +19,7 @@
 $(BINS): %: %.o $(EXTRA_OBJS)
 	$(CC) $(CFLAGS) $(LDFLAGS) -o $@$(BIN_SUFFIX) $^ $(LDLIBS)
 
-install-bins:
+install-bins: $(BINS)
 	@mkdir -p $(DESTDIR)$(sbindir)
 	@for b in $(BINS); do \
 		install $$b$(BIN_SUFFIX) $(DESTDIR)$(sbindir) || exit 1; \