PR25118: move system_header pragma before uses of include_next to avoid extension warnings for people finding libc++ headers via -I paths.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@249788 91177308-0d34-0410-b5e6-96231b3b80d8

include/errno.h

@@ -24,12 +24,13 @@ Macros:
 */
 
 #include <__config>
-#include_next <errno.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
+#include_next <errno.h>
+
 #if !defined(EOWNERDEAD) || !defined(ENOTRECOVERABLE)
 
 #ifdef ELAST