Fix Metal includes breaking macOS local builds

Before this fix, skia (and thus Chrome) would fail to compile on macOS when the user (developer) had a case-sensitive file-system.

So I've replaced the incorrect includes of <metal/metal.h> by <Metal/Metal.h>

Change-Id: I6ebcc0f46608f6d840d80d18e5f5baf0744a7f16
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/225776
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/src/gpu/mtl/GrMtlGpuCommandBuffer.h b/src/gpu/mtl/GrMtlGpuCommandBuffer.h
index 7e820e3..3bf1af5 100644
--- a/src/gpu/mtl/GrMtlGpuCommandBuffer.h
+++ b/src/gpu/mtl/GrMtlGpuCommandBuffer.h
@@ -13,7 +13,7 @@
 #include "src/gpu/GrOpFlushState.h"
 #include "src/gpu/mtl/GrMtlGpu.h"
 
-#import <metal/metal.h>
+#import <Metal/Metal.h>
 
 typedef uint32_t GrColor;
 class GrMtlBuffer;