kbuild: Enable -Wenum-conversion by default

kbuild: Enable -Wenum-conversion by default

This diagnostic checks whether there is a type mismatch when
converting enums (assign an enum of type A to an enum of type B, for
example) and it caught a legit issue recently. The reason it didn't show
is because that warning is enabled only with -Wextra with GCC. Clang,
however, enables it by default.

GCC folks were considering enabling it by default but it was too noisy
back then:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78736

Now that due to clang all those warnings have been fixed, enable it with
GCC too.

allmodconfig tests done with: x86, arm{,64}, powerpc{,64}, riscv
crossbuilds.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>