arpa/inet: fix mingw compilation warning

arpa/inet: fix mingw compilation warning

Mingw doesn't have <arpa/inet.h>, so a conditional use of the
pragma was never encountered, and led to mingw's gcc issuing
warnings about duplicate declarations under -Wredundant-decls.
Making the #pragma unconditional (as was already done in at
least fcntl.in.h) shuts up gcc.

* lib/arpa_inet.in.h (system_header): Hoist to be unconditional.
Reported by Matthew Bolte.

Signed-off-by: Eric Blake <eblake@redhat.com>