srt: Fix autotools build

0a350c61 broke the build by only
building enum types with meson. It also removed gstsrt.c from the list
of sources, causing the plugin to fail to load.

squash! srt: Fix autotools build
8 jobs for srt-autotools in 19 minutes and 21 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #80446
gst indent

00:00:35

passed #80445
manifest

00:00:21

 
  Build
passed #80447
build fedora x86_64

00:05:45

 
  Test
passed #80448
check fedora

00:03:36

 
  Full Builds
passed #80449
build android arm64 api28

00:04:28

passed #80451
build cerbero cross win32

00:08:56

passed #80452
build cerbero cross win64

00:09:07

passed #80450
build cerbero fedora x86_64

00:09:23