fix compatibility features.h for Fedora 34

Recently (for example in Fedora 34), the C++ header files started to
include the system's <single_threaded.h>, and that include <features.h>
and assumes it is enough to define __BEGIN_DECLS. However, this macro
is actually defined in <sys/cdefs.h>, so we get many build errors like:

/usr/include/sys/single_threaded.h:24:1: error: ‘__BEGIN_DECLS’ does not name a type
   24 | __BEGIN_DECLS
      | ^~~~~~~~~~~~~

The fix is to include <sys/cdefs.h> from our compatibility <features.h>.
This is what glibc does too.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20210614062057.1998552-2-nyh@scylladb.com>

include/glibc-compat/features.h

@@ -10,6 +10,11 @@
 
 #include_next <features.h>
 
+/* Recently, some system header files started to rely on <features.h>
+ * already including <sys/cdefs.h>. So we need to do it.
+ */
+#include <sys/cdefs.h>
+
 #define hidden __attribute__((__visibility__("hidden")))
 
 #define __GNU_LIBRARY__ 6