xen/gunzip: Fix build with clang after 33bc2a8495f7

xen/gunzip: Fix build with clang after 33bc2a8495f7

The compilation will fail when building Xen with clang and
CONFIG_DEBUG=y:

make[4]: Leaving directory '/oss/xen/xen/common/libelf'
  INIT_O  gunzip.init.o
Error: size of gunzip.o:.text is 0x00000019

This is because the function init_allocator() will not be inlined
and is not part of the init section.

Fix it by marking init_allocator() with INIT.

Fixes: 33bc2a8495f7 ("xen/gunzip: Allow perform_gunzip() to be called multiple times")
Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Acked-by: Jan Beulich <jbeulich@suse.com>