efivar: fix build with gcc 7

Add upstream patch fixing a warning that breaks the build because of -Werror.

Fixes:
http://autobuild.buildroot.net/results/33a/33adc3ef139d6814aef4c92ae0bcc4c810ab0b86/
http://autobuild.buildroot.net/results/e7d/e7d80e823e13edc6698148244553bd90367bcd03/
http://autobuild.buildroot.net/results/3b6/3b61246f8b04a332d1c61732f0eb6e50ea8ca366/



Cc: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>