motion: fix static link with gnutls

base64_encode is already defined in gnutls so rename it as
motion_base64_encode to prevent a definition clash when linking
statically with gnutls

Fixes:
  - http://autobuild.buildroot.org/results/592672b8826f4c731c50d29725da964d876573c4



Signed-off-by: Fabrice Fontaine <fabrice.fontaine@orange.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>