lavu/timestamp: Avoid C++-unfriendly code in user header

Including this header in a C++11 program (inside extern "C") will
throw an error because it looks like a user-defined literal.  Add a
space between the two tokens to avoid the problem.

libavutil/timestamp.h

@@ -43,7 +43,7 @@
 static inline char *av_ts_make_string(char *buf, int64_t ts)
 {
     if (ts == AV_NOPTS_VALUE) snprintf(buf, AV_TS_MAX_STRING_SIZE, "NOPTS");
-    else                      snprintf(buf, AV_TS_MAX_STRING_SIZE, "%"PRId64, ts);
+    else                      snprintf(buf, AV_TS_MAX_STRING_SIZE, "%" PRId64, ts);
     return buf;
 }