gauche: fix parallel build

Add a patch to fix parallel build issue on ext/rfc

Fixes:
 - http://autobuild.buildroot.org/results/f4935e29ce6aaebdaa47d46c56120b7e97145d1b



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