xquartz: Remove trailing quote to fix build

Fix xquartz build after typo in 3c78d637 ("global: Remove BUILD_DATE and
BUILD_TIME")
Status Job ID Name Coverage
  Docker Image
passed #821486
debian-testing

00:00:31

 
  Build And Test
passed #821487
autotools-build-and-test

00:03:49

passed #821488
meson-build-and-test

00:01:29

 
  External
passed continuous-integration/appveyor/branch