kmod.mk: Use portable printf '%s' over non-portable echo -n

Whilst /bin/echo on macOS and Linux implement -n, as do the builtin
echos in bash and zsh, the builtin echo in dash does not, causing the
first line of the output to be -n foo rather than just foo, and there to
be an extra newline in the output and thus blank line, both of which
result in "Symbol ... is not present in *.kld" warnings appearing in the
build output (once for -n foo and once for the empty string for each
module where EXPORT_SYMS is a list of symbols).

MFC after: 1 week