meson: Fix libshadow.so linkage

Don't link against fb, it's the driver's responsibility to load that
first. Underlinking like this is unpleasant but this matches what
autotools does.

Fixes: xorg/xserver#540
3 jobs for fix-libshadow in 8 minutes and 25 seconds (queued for 17 seconds)
Status Job ID Name Coverage
  Docker Image
passed #411848
debian-testing

00:00:31

 
  Build And Test
passed #411849
autotools-build-and-test

00:06:08

passed #411850
meson-build-and-test

00:01:45