compile-libvlc.sh: fix NDK version check

The diagnostic certainly implies that the shell script shall abort if
the version of the NDK is less than 11, though the previous case-check
only matched versions matching "11*".

This commit changes the implementation so that we check whether "$REL"
is greater-than-or-equal compared to 11.

Signed-off-by: Thomas Guillem <thomas@gllm.fr>