meson: Fix build on {cross-,}win{32,64}

We use visual studio module definitions for the list of symbols to
export when targetting Windows. Fixes CI failure:

../validate/tools/gst-validate.c:460: undefined reference to `gst_validate_spin_on_fault_signals'
4 jobs for vs-module-defs-missing-symbol in 6 minutes and 37 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed gst indent #59442

00:00:21

passed manifest #59441

00:00:24

 
  Build
passed build fedora x86_64 #59443

00:03:06

 
  Full Builds
passed build android arm64 api28 #59444

00:03:05