ipxe: Inhibit -Werror to fix build with gcc-11

* with gcc-11 it fails with:
  drivers/net/ath/ath5k/ath5k_eeprom.c:437:65: error: 'val' may be used uninitialized [-Werror=maybe-uninitialized]
    437 |                 ee->ee_switch_settling_turbo[mode] = (val >> 8) & 0x7f;
        |                                                      ~~~~~~~~~~~^~~~~~

  tests/bigint_test.c: In function 'bigint_test_exec':
  tests/bigint_test.c:232:14: error: 'result_raw' may be used uninitialized [-Werror=maybe-uninitialized]
    232 |         ok ( memcmp ( result_raw, expected_raw,                         \
        |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    233 |                       sizeof ( result_raw ) ) == 0 );                   \
        |                       ~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>