Makefile: add drk binary in variable BINS and reorg clean

For the reorganisation of the clean target, following the order in the variable
BINS.

Makefile

@@ -21,6 +21,7 @@ BINS = \
 	darkfid \
 	darkfi-mmproxy \
 	minerd \
+	drk \
 	darkirc \
 	genev \
 	genevd \
@@ -163,8 +164,9 @@ clean:
 	$(MAKE) -C src/contract/deployooor clean
 	$(MAKE) -C bin/zkas clean
 	$(MAKE) -C bin/darkfid clean
-	$(MAKE) -C bin/minerd clean
 	$(MAKE) -C bin/darkfi-mmproxy clean
+	$(MAKE) -C bin/minerd clean
+	$(MAKE) -C bin/drk clean
 	$(MAKE) -C bin/darkirc clean
 	$(MAKE) -C bin/genev/genev-cli clean
 	$(MAKE) -C bin/genev/genevd clean