Make complex.h play nice with C

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@109791 91177308-0d34-0410-b5e6-96231b3b80d8

include/complex.h

@@ -18,8 +18,16 @@
 
 */
 
+#ifdef __cplusplus
+
 #include <ccomplex>
 
+#else
+
+#include_next <complex.h>
+
+#endif
+
 #pragma GCC system_header
 
 #endif  // _LIBCPP_COMPLEX_H