graphics/qt5-imageformats: prepare for graphics/jsaper update

The build fails on FreeBSD 12.3 due to undefined max_align_t:

In file included from test.cc:2:
In file included from /usr/local/include/jasper/jasper.h:78:
In file included from /usr/local/include/jasper/jas_init.h:73:
In file included from /usr/local/include/jasper/jas_malloc.h:81:
In file included from /usr/local/include/jasper/jas_thread.h:84:
In file included from /usr/include/c++/v1/stdlib.h:100:
In file included from /usr/include/c++/v1/math.h:311:
In file included from /usr/include/c++/v1/type_traits:417:
/usr/include/c++/v1/cstddef:55:9: error: expected unqualified-id
using ::max_align_t;

		^

Include cstddef to make it available.

PR: 262039