avcodec/ttmlenc: Deduplicate ttml_default_namespacing string

String literals are allowed to be deduplicated (and toolchains
are already capable of doing so), yet the same is not allowed
for named arrays (even when they contain strings). Therefore
use a const char *const pointing to an unnamed string literal
for ttml_default_namespacing.

Reviewed-by: Jan Ekström <jeebjp@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/ttmlenc.h

@@ -25,7 +25,7 @@
 #define TTMLENC_EXTRADATA_SIGNATURE "lavc-ttmlenc"
 #define TTMLENC_EXTRADATA_SIGNATURE_SIZE (sizeof(TTMLENC_EXTRADATA_SIGNATURE) - 1)
 
-static const char ttml_default_namespacing[] =
+static const char *const ttml_default_namespacing =
 "  xmlns=\"http://www.w3.org/ns/ttml\"\n"
 "  xmlns:ttm=\"http://www.w3.org/ns/ttml#metadata\"\n"
 "  xmlns:tts=\"http://www.w3.org/ns/ttml#styling\"\n"