Makefile: run $(nproc) parallel jobs for target 'deps'

Change-Id: I53112a20d78d12bcc2764c1fc5567eb26bfa46cc
diff --git a/Makefile b/Makefile
index 87ce759..91c826c 100644
--- a/Makefile
+++ b/Makefile
@@ -65,7 +65,7 @@
 
 .PHONY: deps
 deps:
-	$(MAKE) -C deps
+	$(MAKE) $(PARALLEL_MAKE) -C deps
 
 # deps-update target for backwards compat; now does the same as 'make deps'
 .PHONY: deps-update