Fix missing gc root in compile-all (#25128)
```
/home/keno/julia/src/precompile.c:191:5: note: GC frame changed here
JL_GC_PUSH2(&p, &methsig);
^~~~~~~~~~~~~~~~~~~~~~~~~
/home/keno/julia/src/precompile.c:193:5: note: Loop condition is true. Entering loop body
while (!incr) {
^
/home/keno/julia/src/precompile.c:194:24: note: Started tracking value here
jl_svec_t *p = jl_alloc_svec_uninit(l);
^~~~~~~~~~~~~~~~~~~~~~~
/home/keno/julia/src/precompile.c:195:9: note: Loop condition is true. Entering loop body
for (i = 0, idx_ctr = 0, incr = 1; i < l; i++) {
^
/home/keno/julia/src/precompile.c:197:13: note: Taking false branch
if (jl_is_uniontype(ty)) {
^
/home/keno/julia/src/precompile.c:195:9: note: Loop condition is false. Execution continues on line 217
for (i = 0, idx_ctr = 0, incr = 1; i < l; i++) {
^
/home/keno/julia/src/precompile.c:217:32: note: Passing non-rooted value as argument to function that may GC
methsig = (jl_value_t*)jl_apply_tuple_type(p);
^ ~
```
Essentially the same bug as #25123
