*: use SWAP_BE64 instead of open-coding it

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
diff --git a/include/libbb.h b/include/libbb.h
index dec4e3a..b16157d 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -1540,7 +1540,7 @@
 	uint32_t C;
 	uint32_t D;
 	uint64_t total64;
-	char wbuffer[64];
+	char wbuffer[64]; /* NB: always correctly aligned for uint64_t */
 } md5_ctx_t;
 #else
 /* libbb/md5prime.c uses a bit different one: */