fix(audio): use FFMPEG (all-caps) to match vcpkg Find module file name

vcpkg provides FindFFMPEG.cmake (all-caps), but find_package(FFmpeg)
(mixed-case) looks for FindFFmpeg.cmake on case-sensitive Linux.
Windows works due to case-insensitive filesystem. Changing to
find_package(FFMPEG) makes the Find module name match on all platforms.

Author: wolfgitpr

lib/Audio/CMakeLists.txt

@@ -18,7 +18,7 @@ file(GLOB_RECURSE _src CONFIGURE_DEPENDS
 )
 
 find_package(stdcorelib CONFIG REQUIRED)
-find_package(FFmpeg REQUIRED)
+find_package(FFMPEG REQUIRED)
 find_package(SndFile CONFIG REQUIRED)
 
 synthrt_add_library(${PROJECT_NAME} SHARED PREFIX srt_audio