go-fuzz-build: ignore new runtime internal package
go-fuzz-build: ignore new runtime internal package

Added recently to Go tip. Without this line, the build program breaks as
expected:

	$ go-fuzz-build mvdan.cc/sh
	failed to execute go build: exit status 1
	import cycle not allowed
	package mvdan.cc/sh/go.fuzz.main
		imports go-fuzz-dep
		imports runtime
		imports runtime/internal/math
		imports go-fuzz-dep

dvyukov/go-fuzz,dvyukov/go-fuzz