libjulia: add jl_unwrap_unionall to exported_funcs

[benlorenz]

This was recently refactored in #51319 and while trying to adapt libcxxwrap-julia to the new changes I got:

ld: CMakeFiles/test_module.dir/test_module.cpp.o: in function `jl_datatype_layout':
test_module.cpp:(.text+0x2c2): undefined reference to `jl_unwrap_unionall'

The function jl_datatype_layout is STATIC_INLINE in julia.h:

julia/src/julia.h

Lines 1304 to 1312 in 98542d7

	JL_DLLEXPORT jl_value_t *jl_unwrap_unionall(jl_value_t *v JL_PROPAGATES_ROOT) JL_NOTSAFEPOINT;
	#define jl_inlinedatatype_layout(t) (((jl_datatype_t*)t)->layout)
	STATIC_INLINE const jl_datatype_layout_t *jl_datatype_layout(jl_datatype_t *t) JL_NOTSAFEPOINT
	{
	if (jl_is_layout_opaque(t->layout)) // e.g. GenericMemory
	t = (jl_datatype_t*)jl_unwrap_unionall(t->name->wrapper);
	return t->layout;
	}

jl_unwrap_unionall is marked JL_DLLEXPORT here but it doesn't appear in the exports of libjulia.