gst-gl: Remove duplicate declarations

This causes 'redefinition of typedef ...' errors on GCC 4.5.3
3 jobs for milloni-ct/gcc-4-9 in 5 minutes and 5 seconds (queued for 3 minutes and 14 seconds)
Status Name Job ID Coverage
  Preparation
passed manifest #52067

00:00:37

 
  Build
passed build fedora x86_64 #52068

00:01:36

 
  Full Builds
passed build android arm64 api28 #52069

00:02:51