ipxe: fix gcc5 build

gcc5 enables additional warnings, causing the build to fail with
-Werror.  The build could be fixed by specifically disabling errors
for `discarded-array-qualifiers` and `logical-not-parentheses` warnings,
but simply passing -Wno-error is more future proof.

See https://hydra.nixos.org/build/33274006/nixlog/1/raw