tools/mpy_ld.py: Pre-declare some local variables to appease linter.

Spurious fix as the logic is structured such that these variables will be
set before dereferenced, but this keeps Ruff happy (no more F821
undefined-name).

Signed-off-by: Angus Gratton <angus@redyak.com.au>

tools/mpy_ld.py

@@ -966,6 +966,9 @@ def build_mpy(env, entry_offset, fmpy, native_qstr_vals, native_qstr_objs):
 
     # MPY: relocation information
     prev_kind = None
+    prev_base = None
+    prev_offset = None
+    prev_n = None
     for base, addr, kind in env.mpy_relocs:
         if isinstance(kind, str) and kind.startswith(".text"):
             kind = 0