Fix missing gc root in struct allocation (#25124)
ClangSA says:
```
/home/keno/julia/src/datatype.c:729:22: note: Started tracking value here
jl_value_t *jv = jl_gc_alloc(ptls, jl_datatype_size(type), type);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./julia_internal.h:285:6: note: expanded from macro 'jl_gc_alloc'
(jl_gc_alloc)(ptls, sz, ty))
^~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/keno/julia/src/datatype.c:730:24: note: Assuming 'i' is < 'na'
for (size_t i = 0; i < na; i++) {
^~~~~~
/home/keno/julia/src/datatype.c:730:5: note: Loop condition is true. Entering loop body
for (size_t i = 0; i < na; i++) {
^
/home/keno/julia/src/datatype.c:732:14: note: Value may have been GCed here
if (!jl_isa(args[i], ft))
^~~~~~~~~~~~~~~~~~~
/home/keno/julia/src/datatype.c:732:13: note: Assuming the condition is false
if (!jl_isa(args[i], ft))
^~~~~~~~~~~~~~~~~~~~
/home/keno/julia/src/datatype.c:732:9: note: Taking false branch
if (!jl_isa(args[i], ft))
^
/home/keno/julia/src/datatype.c:734:9: note: Argument value may have been GCed
jl_set_nth_field(jv, i, args[i]);
^ ~~
```
I concur.
