ljsyscall: move installation path to /usr/share/lua/5x

Move ljsyscall install destination to /usr/share/lua/$(abiver}, to
match what the luarocks install would naturally do.  This also
reflects the value of INSTALL_LMOD in the luajit.pc file.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Tested-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>