guile: add patch borrowed from Yocto to fix compiler error

Fixes:

  http://autobuild.buildroot.org/results/85b/85bd849e17c6e5ed6b5b56a4d4ad1a73f7de99c2/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>