opusenc: Fix -Wstrict-prototypes

In preparation for C23. Fixes the following warning with Clang 16:
```
src/opusenc.c:82:37: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
OggOpusComments *ope_comments_create() {
                                    ^
                                     void
```

Bug: https://bugs.gentoo.org/879701
Signed-off-by: Sam James <sam@gentoo.org>
Signed-off-by: Mark Harris <mark.hsj@gmail.com>

src/opusenc.c

@@ -79,7 +79,7 @@ struct OggOpusComments {
 };
 
 /* Create a new comments object. The vendor string is optional. */
-OggOpusComments *ope_comments_create() {
+OggOpusComments *ope_comments_create(void) {
   OggOpusComments *c;
   const char *libopus_str;
   char vendor_str[1024];