kbuild: drop -Wundef from HOSTCFLAGS for now

-Wundef caused warnings in the bison generated code in kconfig.
Updating to a newer bison (1.875d) did not fix it. The alternatives
was to correct the autogenerated code or drop -Wundef.
For now -Wundef is dropped from HOSTCFLAGS.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
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