With the addition of gcc 9 recipes it highlighted there is no PREFERRED_VERSION
set for libgfortran and it should match the rest of gcc. Add this missing
PREFERRED_VERSION line to avoid mixing gcc versions in inadvisable ways.
(From OE-Core rev: 172dfdd62b012dc63f492d33239af209f83f4966)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>