Added a target for 'make clean'

Makefile

@@ -12,4 +12,5 @@ demo.o: demo.c bsspeke.h
 demo: demo.o bsspeke.o monocypher.o
 	cc -o demo demo.o bsspeke.o monocypher.o
 
-
+clean:
+	rm -f demo bsspeke.o monocypher.o