Use 'libopus' rather than 'Opus' in the version string.

This better distinguishes the reference implementation,
which is what this string is meant to do. Thanks to
Greg Maxwell for the suggestion.

src/opus.c

@@ -32,7 +32,7 @@
 #include "opus.h"
 
 #ifndef OPUS_VERSION
-#define OPUS_VERSION "Opus version unknown"
+#define OPUS_VERSION "libopus version unknown"
 #endif
 
 const char *opus_strerror(int error)
@@ -55,5 +55,5 @@ const char *opus_strerror(int error)
 
 const char *opus_get_version_string(void)
 {
-	return "Opus " OPUS_VERSION;
+	return "libopus " OPUS_VERSION;
 }