ftp/R-cran-curl: fix build on big-endian architectures

BSWAP_32 is undefined on FreeBSD, use bswap32.

It also needs sys/endian.h include.

Needed because of:

/wrkdirs/usr/ports/ftp/R-cran-curl/work/stage/usr/local/lib/R/library/00LOCK-curl/00new/curl/libs/curl.so: Undefined symbol "BSWAP_32"

PR: 238704
Approved by: tota (maintainer), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D20726