package/bootstrap: fix licence file path

The licence file is in css/ directory.

Fixes:
http://autobuild.buildroot.net/results/0d4/0d4b9488d1aa80e8d290e4dfe1937c3879eceea4



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>