libstdc++: use extern "C++" in all our C++ system headers.

This is needed to build an independent toolchain with g++ that doesn't think
that all these headers are in C.

Change-Id: Ie9a8ccfcab7780d6a4e5722777d61c2b1b312001
diff --git a/libstdc++/include/ctime b/libstdc++/include/ctime
index 36fe7d5..9e6744f 100644
--- a/libstdc++/include/ctime
+++ b/libstdc++/include/ctime
@@ -37,6 +37,8 @@
 #include <cstddef>
 #include <time.h>
 
+extern "C++" {
+
 namespace std 
 {
 // Types.
@@ -56,4 +58,6 @@
 using ::strftime;
 }  // namespace std
 
+}  // extern C++
+
 #endif  // BIONIC_LIBSTDCPP_INCLUDE_CTIME__