endian.h: fix so that this builds on NetBSD again.

[he32]

A little ironic, since parts of this code is from NetBSD.

[he32]

Previous build error was:

In file included from /usr/include/stdarg.h:37,
                 from ./include/zone.h:17,
                 from ./src/fallback/parser.c:9:
./src/generic/endian.h:125:31: error: expected declaration specifiers or '...' before '__builtin_constant_p'
  125 | static really_inline uint16_t bswap16(uint16_t x)
      |                               ^~~~~~~
./src/generic/endian.h:133:31: error: expected declaration specifiers or '...' before '__builtin_constant_p'
  133 | static really_inline uint32_t bswap32(uint32_t x)
      |                               ^~~~~~~
./src/generic/endian.h:144:31: error: expected declaration specifiers or '...' before '__builtin_constant_p'
  144 | static really_inline uint64_t bswap64(uint64_t x)
      |                               ^~~~~~~

[k0ekk0ek]

Thanks @he32!

[obache]

This issue should be already resolved with #231, so this change is not required.

[k0ekk0ek]

I'll have a look next week @obache (created #234). Thanks 👍