read-edid: fix build on -fno-common toolchains

Without the change build fails on toolchains like clang-13 which
switch to -fno-common by default:

    $ nix build -L --impure --expr 'with import ./. {}; read-edid.override { stdenv = clang13Stdenv; }'
    ...
    read-edid> ld: CMakeFiles/get-edid.dir/classic.c.o:(.bss+0x0):
      multiple definition of `quiet'; CMakeFiles/get-edid.dir/get-edid.c.o:(.bss+0x0): first defined here
    read-edid> ld: CMakeFiles/get-edid.dir/i2c.c.o:(.bss+0x0):
      multiple definition of `quiet'; CMakeFiles/get-edid.dir/get-edid.c.o:(.bss+0x0): first defined here

0 files changed, 0 insertions, 0 deletions