gschema.dtd: Add target attribute to alias

The implementation requires it, but the DTD doesn’t even mention it.
7 jobs for backport-814-gschema-fix-glib-2-60 in 16 minutes (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #293221
cross-android_api21_arm64

00:01:50

passed #293222
cross-android_api28_arm64

00:01:51

passed #293223
cross-mingw64

00:02:14

passed #293220
fedora-x86_64

00:05:26

passed #293224
win32
msys2-mingw32

00:13:50

passed #293225
win32
vs2017-x64

00:08:54

 
  Coverage
passed #293226
coverage

00:02:08

74.7%