journal/lookup3: silence gcc 7 implicit-fallthrough warning

This file doesn't include any of our headers, so just use the pragma
without defining it in macros.h

src/journal/lookup3.c

@@ -48,6 +48,10 @@ on 1 byte), but shoehorning those bytes into integers efficiently is messy.
 # include <endian.h>    /* attempt to define endianness */
 #endif
 
+#if __GNUC__ >= 7
+_Pragma("GCC diagnostic ignored \"-Wimplicit-fallthrough\"")
+#endif
+
 /*
  * My best guess at if you are big-endian or little-endian.  This may
  * need adjustment.