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

llvm-svn: 249788
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 2999ea0f041659f6ec0cee0fa6ab119c382bfac2
diff --git a/include/math.h b/include/math.h
index dc05622..2020554 100644
--- a/include/math.h
+++ b/include/math.h
@@ -293,12 +293,13 @@
 */
 
 #include <__config>
-#include_next <math.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
+#include_next <math.h>
+
 #ifdef __cplusplus
 
 // We support including .h headers inside 'extern "C"' contexts, so switch