BUILD: hpack: hpack-tbl-t.h uses VAR_ARRAY but does not include compiler.h

BUILD: hpack: hpack-tbl-t.h uses VAR_ARRAY but does not include compiler.h

This fixes building hpack from contrib, which failed because of the
undeclared VAR_ARRAY:

make -C contrib/hpack
...
cc -O2 -Wall -g -I../../include -fwrapv -fno-strict-aliasing   -c -o gen-enc.o gen-enc.c
In file included from gen-enc.c:18:
../../include/haproxy/hpack-tbl-t.h:105:23: error: 'VAR_ARRAY' undeclared here (not in a function)
  105 |  struct hpack_dte dte[VAR_ARRAY]; /* dynamic table entries */
...

As discussed in the thread below, let's redefine VAR_ARRAY in this file
so that it remains self-sustaining:

   https://www.mail-archive.com/haproxy@formilux.org/msg39212.html