package/mbuffer: bump to version 20200505

This bump will fix a build failure with gcc 4.8

Fixes:
 - http://autobuild.buildroot.org/results/1a7fe7ac8cbe9997bc5d1e9ade24bc2ee6ac6abe



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>