Fix crash when compiling division by a power of 2 from BPF

Due to a typo pflua currently crashes when compiling from BPF if there is a division by a power of two.

Testcase:

```bash
$ cd src
$ ../deps/luajit/usr/local/bin/luajit ../tools/pflang-compile --bpf-lua 'tcp and (ip[0]/4 != 0)'
```

Result before fix:

```
../deps/luajit/usr/local/bin/luajit: ./pf/bpf.lua:250: attempt to call global 'shr' (a nil value)
stack traceback:
	./pf/bpf.lua:250: in function 'alu'
	./pf/bpf.lua:317: in function 'compile_filter'
	../tools/pflang-compile:47: in main chunk
	[C]: at 0x0804c020
```

Result after fix:

```
return function (P, length)
   local A = 0
   if 14 > length then return 0 end
   A = bit.bor(bit.lshift(P[12], 8), P[12+1])
   if (A==34525) then goto L8 end
   if not (A==2048) then goto L8 end
   if 24 > length then return 0 end
   A = P[23]
   if not (A==6) then goto L8 end
   if 15 > length then return 0 end
   A = P[14]
   A = bit.rshift(A, 2)
   if (A==0) then goto L8 end
   do return 65535 end
   ::L8::
   do return 0 end
   error("end of bpf")
end
```