Fix a warning on Linux Alpine

Fix a warning on Linux Alpine

The warning was like:
In file included from ./netdissect-stdinc.h:42,
                 from ./fptype.c:36:
./ftmacros.h:116: warning: "_BSD_SOURCE" redefined
  116 |   #define _BSD_SOURCE
      |
In file included from /usr/include/stdio.h:8,
                 from /usr/include/fortify/stdio.h:22,
                 from ./fptype.c:34:
/usr/include/features.h:15: note: this is the location of the previous
  definition
   15 | #define _BSD_SOURCE 1
      |