Fix parallel build failure bya dding the dependency.

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=996964
--- a/Makefile
+++ b/Makefile
@@ -6,7 +6,7 @@ debian: cramfsswap
 cramfsswap: cramfsswap.c
 	$(CC) -Wall -g -O $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) -o cramfsswap cramfsswap.c -lz
 
-strip:
+strip: cramfsswap
 	strip cramfsswap
 
 install: cramfsswap