ffmpeg_dec: bump ffmpeg to sof-hifi 181d38ad (clang float_dsp build fix)

Pick up the __XCC__ gate on the Xtensa float_dsp SIMD path so the ffmpeg_dec
module builds cleanly with the LLVM Xtensa clang (was: intrinsic branch entered
via the xt_hifi4.h wrapper and failed on undeclared xtfloatx2). GCC behaviour is
unchanged (scalar fallback either way).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: lrgirdwo

west.yml

@@ -72,7 +72,7 @@ manifest:
     - name: ffmpeg
       repo-path: FFmpeg
       path: modules/audio/ffmpeg
-      revision: a600a78acded219a98c426af0e353e6f2d689b9d
+      revision: 181d38ad362cb22afac3fbd7207d29f7239212b3
       remote: ffmpeg
 
     # libshine: small fixed-point MP3 encoder for FFMPEG_ENC_MP3 (FFmpeg has no