package/elf2flt: fix build with Binutils >= 2.31

Binutils added diagnostics.h to bfd.h [1], so elf2flt must create a
symlink for this header.

Fixes:
https://gitlab.com/free-electrons/toolchains-builder/-/jobs/82284139

[1] https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commitdiff;h=95da9854466ada2572b42f5528711a06a2d42db1



Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>