kbuild: deb-pkg: get rid of |flex:native workaround from Build-Depends

"| flex:native" was a workaround (suggested by Ben, see Link) because
"MultiArch: foreign" was missing in the flex package on some old distros
when commit e3a22850 ("deb-pkg: generate correct build dependencies")
was applied.

It seems fixing the flex package has been completed. Get rid of the
workaround.

Link: https://lore.kernel.org/linux-kbuild/ab49b0582ef12b14b1a68877263b81813e2492a2.camel@decadent.org.uk/
Link: https://wiki.debian.org/CrossBuildPackagingGuidelines


Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Ben Hutchings <ben@decadent.org.uk>