tools/cppcheck/suppressions.txt: ignore errors in gcc-plugin code

tools/cppcheck/suppressions.txt

@@ -62,5 +62,6 @@ allocaCalled:gcc-plugin/src/*
 unusedVariable:gcc-plugin/tests/*
 unreadVariable:gcc-plugin/tests/*
 duplicateExpression:gcc-plugin/src/*
+negativeIndex:gcc-plugin/src/*
 
 pointerSize:socl/src/cl_getcontextinfo.c:33