testing/zile: use -Wno-incompatible-pointer-types to build with gcc 14

The rebuild errors appear to be coming from Vala-generated code. It is probably better to wait for an upstream resolution. Temporarily use -Wno-incompatible-pointer-types in the meantime to rebuild and unblock the builder.

Example error:

src/eval.vala: In function 'minibuf_read_function_name':
src/eval.vala:87:39: error: passing argument 2 of 'g_hash_table_foreach' from incompatible pointer type [-Wincompatible-pointer-types]
   87 |         LispFunc.table.@foreach ((name, f) => {
      |                                       ^~~~~~~~~           
      |                                       |
      |                                       void (*)(const void *, const void *, void *)
In file included from /usr/include/glib-2.0/glib.h:52:
/usr/include/glib-2.0/glib/ghash.h:114:61: note: expected 'GHFunc' {aka 'void (*)(void *, void *, void *)'} but argument is of type 'void (*)(const void *, const void *, void *)'
  114 |                                             GHFunc          func,
      |                                             ~~~~~~~~~~~~~~~~^~~~