Fix warnings introduced in #4852.

[Ms2ger]

No description provided.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3933

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[jdm]

r+

[bors-servo]

saw approval from jdm
at Ms2ger@d5ec37e

[bors-servo]

merging Ms2ger/servo/warnings-4852 = d5ec37e into auto

[bors-servo]

Ms2ger/servo/warnings-4852 = d5ec37e merged ok, testing candidate = 63f6601

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/710
success: http://build.servo.org/builders/linux2/builds/709
success: http://build.servo.org/builders/android/builds/745
success: http://build.servo.org/builders/mac1/builds/926
success: http://build.servo.org/builders/mac2/builds/926

[bors-servo]

fast-forwarding master to auto = 63f6601