sys/src/cmd/disk/prep: fix missing =

sys/src/cmd/disk/prep/prep.c

@@ -269,9 +269,9 @@ static char isfrog[256]={
 	/*BKS*/	1, 1, 1, 1, 1, 1, 1, 1,
 	/*DLE*/	1, 1, 1, 1, 1, 1, 1, 1,
 	/*CAN*/	1, 1, 1, 1, 1, 1, 1, 1,
-	[' ']	1,
-	['/']	1,
-	[0x7f]	1,
+	[' ']	= 1,
+	['/']	= 1,
+	[0x7f]	= 1,
 };
 
 static char*