Build for macOS 15

[awni]

Tests clear with:

• macOS 15
• Xcode 16.0
• macOS SDK 15.0
• Apple clang version 16.0.0

Compiler warnings:

/Users/awnihannun/mlx/mlx/backend/accelerate/matmul.cpp:110:9: warning: 'BNNSLayerParametersBroadcastMatMul' is deprecated: first deprecated in macOS 15.0 - Use BNNSGraph* APIs [-Wdeprecated-declarations]
  110 |   const BNNSLayerParametersBroadcastMatMul gemm_params{
      |         ^
/Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.0.sdk/System/Library/Frameworks/vecLib.framework/Headers/BNNS/bnns_structures.h:1391:3: note: 'BNNSLayerParametersBroadcastMatMul' has been explicitly marked deprecated here
 1391 | } BNNSLayerParametersBroadcastMatMul
      |   ^
/Users/awnihannun/mlx/mlx/backend/accelerate/matmul.cpp:178:7: warning: 'BNNSFilterCreateLayerBroadcastMatMul' is deprecated: first deprecated in macOS 15.0 - Use BNNSGraph* APIs [-Wdeprecated-declarations]
  178 |       BNNSFilterCreateLayerBroadcastMatMul(&gemm_params, nullptr);
      |       ^
/Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.0.sdk/System/Library/Frameworks/vecLib.framework/Headers/BNNS/bnns.h:269:12: note: 'BNNSFilterCreateLayerBroadcastMatMul' has been explicitly marked deprecated here
  269 | BNNSFilter BNNSFilterCreateLayerBroadcastMatMul(const BNNSLayerParametersBroadcastMatMul *layer_params,
      |            ^
/Users/awnihannun/mlx/mlx/backend/accelerate/matmul.cpp:181:5: warning: 'BNNSFilterApplyTwoInput' is deprecated: first deprecated in macOS 15.0 - Use BNNSGraph* APIs [-Wdeprecated-declarations]
  181 |     BNNSFilterApplyTwoInput(
      |     ^
/Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.0.sdk/System/Library/Frameworks/vecLib.framework/Headers/BNNS/bnns.h:541:5: note: 'BNNSFilterApplyTwoInput' has been explicitly marked deprecated here
  541 | int BNNSFilterApplyTwoInput(BNNSFilter filter,
      |     ^
/Users/awnihannun/mlx/mlx/backend/accelerate/matmul.cpp:190:3: warning: 'BNNSFilterDestroy' is deprecated: first deprecated in macOS 15.0 - Use BNNSGraph* APIs [-Wdeprecated-declarations]
  190 |   BNNSFilterDestroy(bnns_filter);
      |   ^
/Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.0.sdk/System/Library/Frameworks/vecLib.framework/Headers/BNNS/bnns.h:827:6: note: 'BNNSFilterDestroy' has been explicitly marked deprecated here
  827 | void BNNSFilterDestroy(BNNSFilter filter)
      |      ^
4 warnings generated.