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/complex.h

@@ -18,6 +18,12 @@
 
 */
 
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
 #ifdef __cplusplus
 
 #include <ccomplex>
@@ -28,8 +34,4 @@
 
 #endif  // __cplusplus
 
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#pragma GCC system_header
-#endif
-
 #endif  // _LIBCPP_COMPLEX_H