Fix compilation warning in unicode_norm.c

Fix compilation warning in unicode_norm.c

80f8eb7 has introduced in unicode_norm.c some new code that uses
htonl().  On at least some FreeBSD environments, it is possible to find
that this function is undeclared, causing a compilation warning.  It is
worth noting that no buildfarm members have reported this issue.

Instead of adding a new inclusion to arpa/inet.h, switch to use
the equivalent defined in pg_bswap.h, to benefit from any built-in
function if the compiler has one.

Reported-by: Masahiko Sawada
Discussion: http://postgr.es/m/CA+fd4k7D4b12ShywWj=AbcHZzV1-OqMjNe7RZAu+tgz5rd_11A@mail.gmail.com