decoder_openjpeg: Add OPJ_CALLCONV to unique_ptr deleter

[lazka]

Fixes calling convention mismatch in unique_ptr declaration by adding OPJ_CALLCONV to the deleter function pointer type.

This fixes the build for 32bit Windows, where the default calling convention doesn't match OPJ_CALLCONV, which is used by all three functions.

This issue was introduced in 4851124559d2cbccc50 (and later adjusted in 242331c3e74a5ba7c)

Fixes #1262

[lazka]

Only tested against 1.18.2, since master doesn't build right now.

[farindk]

Thank you. I've merged this into master and also v1.18.x.

[lazka]

Thanks!