audio: Rename coreaudio extension to use Objective-C compiler

audio: Rename coreaudio extension to use Objective-C compiler

The coreaudio library includes Objective-C declarations (using the
caret '^' symbol to declare block references [*]). When building
with a C compiler we get:

  [175/839] Compiling C object libcommon.fa.p/audio_coreaudio.c.o
    In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX12.sdk/System/Library/Frameworks/CoreAudio.framework/Headers/CoreAudio.h:18,
                     from ../../audio/coreaudio.c:26:
    /Library/Developer/CommandLineTools/SDKs/MacOSX12.sdk/System/Library/Frameworks/CoreAudio.framework/Headers/AudioHardware.h:162:2: error: expected identifier or '(' before '^' token
      162 | (^AudioObjectPropertyListenerBlock)(    UInt32                              inNumberAddresses,
          |  ^
    FAILED: libcommon.fa.p/audio_coreaudio.c.o

Rename the file to use the Objective-C default extension (.m) so
meson calls the correct compiler.

[*] https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/WorkingwithBlocks/WorkingwithBlocks.html

Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>