build: avoid double / in paths for dtrace
build: avoid double / in paths for dtrace

GYP doesn't resolve or normalize the paths, resulting in the action not
firing if the path contains // so take care to avoid that

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso