[LibWebRTC] Unreviewed build fix after 265790@main

https://bugs.webkit.org/show_bug.cgi?id=258708

Compiler flags were not applied to the list of AVX2 source files because
'webrtc_avx_SOURCES' needs to be evaluated.

* Source/ThirdParty/libwebrtc/CMakeLists.txt:

Canonical link: https://commits.webkit.org/265832@main

Author: dpino

Source/ThirdParty/libwebrtc/CMakeLists.txt

@@ -1866,7 +1866,7 @@ if (WTF_CPU_X86_64 OR WTF_CPU_X86)
         Source/webrtc/modules/audio_processing/aec3/vector_math_avx2.cc
         Source/webrtc/modules/audio_processing/agc2/rnn_vad/vector_math_avx2.cc
     )
-    set_source_files_properties(webrtc_avx_SOURCES PROPERTIES COMPILE_OPTIONS "-mavx2;-mfma")
+    set_source_files_properties(${webrtc_avx_SOURCES} PROPERTIES COMPILE_OPTIONS "-mavx2;-mfma")
     list(APPEND webrtc_SOURCES ${webrtc_avx_SOURCES})
     add_definitions(-DWEBRTC_ENABLE_AVX2)
 endif()