DEV: hpack: fix `trash` build regression

DEV: hpack: fix `trash` build regression

Since 7d84439 ("BUILD: hpack: include global.h for the trash that is needed
in debug mode"), hpack decode tool fails to compile on targets that enable
USE_THREAD. (ie: linux-glibc target as reported by Christian Ruppert)

When building hpack devtool, we are including src/hpack-dec.c as a dependency.
src/hpack-dec.c relies on the global trash whe debug mode is enabled.
But as we're building hpack tool with a limited scope of haproxy
sources, global trash (which is declared in src/chunk.c) is not available.
Thus, src/hpack-dec.c relies on a local 'trash' variable declared within
dev/hpack/decode.c

This used to work fine until 7d84439.
But now that global.h is explicitely included in src/hpack-dec.c,
trash variable definition from decode.c conflicts with the one from global.h:

  In file included from include/../src/hpack-dec.c:35,
                   from dev/hpack/decode.c:87:
  include/haproxy/global.h:52:35: error: thread-local declaration of 'trash' follows non-thread-local declaration
     52 | extern THREAD_LOCAL struct buffer trash;

Adding THREAD_LOCAL attribute to 'decode.c' local trash variable definition
makes the compiler happy again.

This should fix GH issue #2009 and should be backported to 2.7.