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/setjmp.h

@@ -27,12 +27,13 @@ void longjmp(jmp_buf env, int val);
 */
 
 #include <__config>
-#include_next <setjmp.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
+#include_next <setjmp.h>
+
 #ifndef setjmp
 #define setjmp(env) setjmp(env)
 #endif