libdca.recipe: Fix symlink creation code

Fixes CI error:
Traceback (most recent call last):
  File "/builds/nirbheek/cerbero/cerbero/build/oven.py", line 461, in _cook_recipe_step
    ret = stepfunc()
  File "/builds/nirbheek/cerbero/cerbero/build/recipe.py", line 93, in wrapped
    stepfunc()
  File "/builds/nirbheek/cerbero/recipes/libdca.recipe", line 34, in post_install
    os.symlink(src, os.path.join(self.config.prefix, dest))

FileExistsError: [Errno 17] File exists: 'lib/libdts.a' -> '/builds/nirbheek/cerbero/cerbero-build/dist/mingw_x86_64/libdca.a'
Part-of: <!618>
15 jobs for master in 71 minutes and 32 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Build Docker
passed alpine amd64 manifest builder docker #4779662

00:00:11

passed android docker #4779661

00:00:16

passed cerbero fedora amd64 docker #4779660

00:00:12

passed fedora amd64 docker #4779659

00:00:30

passed gst-indent amd64 docker #4779663

00:00:11

 
  Preparation
passed manifest #4779664

00:00:25

 
  Build
passed cerbero deps cross-android universal #4779666
gstreamer

00:17:13

passed cerbero deps cross-ios universal #4779670
gst-ios-13.2

01:09:02

passed cerbero deps cross-windows x86 #4779667
gstreamer

00:07:19

passed cerbero deps cross-windows x86_64 #4779668
gstreamer

00:10:23

passed cerbero deps fedora x86_64 #4779665
gstreamer

00:07:14

passed cerbero deps macos x86_64 #4779669
gst-macos-10.15

00:47:34

passed cerbero deps msvc x86_64 #4779671
docker windows gstreamer-windows 1809

00:47:59

 
  Integrate
passed cerbero cross-android universal examples #4779672

00:05:35

passed cerbero cross-ios universal examples #4779673
gst-ios-13.2

00:01:32