luasocket: add Lua 5.4 variant

luasocket: add Lua 5.4 variant

Add luasocket5.4 next to the existing luasocket (Lua 5.1) and
luasocket5.3 variants, installed via the upstream makefile's
'make install LUAV=5.4'.

That makefile defaults to prefix=/usr/local on linux, which is where
the luasocket5.3 variant still installs, so pass prefix=/usr explicitly
to land in /usr/share/lua/5.4 and /usr/lib/lua/5.4 like the other Lua
5.4 module packages.

Needed for the prosody 13.x upgrade.

Signed-off-by: Alexandru Ardelean <redacted>