meson: Add libsinc to the CFLAGS

Otherwise gst-i18n-plugin.h can't be found.
6 jobs for i18n in 34 minutes (queued for 23 minutes and 23 seconds)
Status Job ID Name Coverage
  Preparation
passed #65147
gst indent

00:00:28

passed #65146
manifest

00:00:24

 
  Build
passed #65148
build fedora x86_64

00:03:51

 
  Test
passed #65149
check fedora

00:15:53

 
  Full Builds
passed #65150
build android arm64 api28

00:03:07

passed #65151
build cerbero fedora x86_64

00:10:14