Compile the HDRP path tracing shader only for the supported d3d platforms

The compilation of the HDRP path tracing shader timeouts on some consoles. The issue can be reproduced just by switching to the PS5 platform.
https://jira.unity3d.com/browse/UUM-72861

Due to hardware/API limitations, the path tracer is only fully functional on d3d platforms right now, so with my PR we only target these platforms.

Author: pmavridis

Packages/com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/PathTracing/Shaders/PathTracingMain.raytrace

@@ -1,6 +1,9 @@
 // We do not rely on recursion, beyond subsurface scattering
 #pragma max_recursion_depth 2
 
+// On some consoles we get a timeout when compiling this, so we only target d3d platforms
+#pragma only_renderers d3d11
+
 // SRP/HDRP includes
 #define SHADER_TARGET 50
 // Include and define the shader pass (Required for light cluster code)