libavutil/error: fix build with musl toolchain

Add the feature test macro which is required for building with the
musl toolchain.

The feature test macro _XOPEN_SOURCE = 600 provides the XSI-compliant
version of strerror_r().
Signed-off-by: Jörg Krause <jkrause@posteo.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/error.c

@@ -17,6 +17,7 @@
  */
 
 #undef _GNU_SOURCE
+#define _XOPEN_SOURCE 600 /* XSI-compliant version of strerror_r */
 #include "avutil.h"
 #include "avstring.h"
 #include "common.h"