Update PassPlugin include (#3477)

Update for llvm-project commit f54df0d09e19 ("[LLVM][NFC] Move
PassPlugin from Passes to separate library", 2025-12-22).

Author: svenvh

lib/SPIRV/PassPlugin.cpp

@@ -49,7 +49,7 @@
 #include "SPIRVWriter.h"
 
 #include "llvm/Passes/PassBuilder.h"
-#include "llvm/Passes/PassPlugin.h"
+#include "llvm/Plugins/PassPlugin.h"
 
 using namespace llvm;