Fix macOS RTP endian macro mapping

[idy]

Summary

• map the endian macros exported by <machine/endian.h> to the underscored identifiers used by libpeer's RTP bit-field selector
• keep the current RTP layout and all non-Apple preprocessing unchanged
• provide a minimal alternative to the broader explicit serialization change in Fix RTP header wire serialization #267

Root cause

On macOS, BYTE_ORDER, BIG_ENDIAN, and LITTLE_ENDIAN are defined, but __BYTE_ORDER, __BIG_ENDIAN, and __LITTLE_ENDIAN are not. Undefined identifiers in the RTP #if expression evaluate as zero, so the big-endian bit-field layout is selected and an Opus RTP v2/PT111 header starts with 02 de instead of 80 6f.

Validation

• compiled src/rtp.c with a strict (-Wall -Wextra -Werror) RTP wire regression on AppleClang
• confirmed the regression fails on upstream main at the expected first-byte assertion and passes with this change
• attempted the full CMake build; local dependency builds are blocked independently by obsolete cJSON/usrsctp CMake policies and existing mbedTLS documentation warnings promoted to errors by the current AppleClang

Fixes #268