perl: fix build caused by bison generated code

This commit makes sure the date of a C file is newer than the date of
the corresponding Bison source file, so that the build process doesn't
try to regenerate it. This avoids the need to have host-bison as a
dependency for the perl build.

Fixes:

  http://autobuild.buildroot.net/results/1c2/1c261b09f5a4a314d5dcbc1e5811af9c7d18658a/



Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>