SingletonVault_c.cpp is always C++

Summary: It's simply not possible to compile the C++ side of the C API as C, so don't guard the `extern "C"` as if it is.

Reviewed By: meyering, yfeldblum

Differential Revision: D3369474

fbshipit-source-id: d3316704ce17a471bc9d6a3a235472bf40cedb9d

folly/SingletonVault_c.cpp

@@ -17,9 +17,7 @@
 #include <folly/SingletonVault_c.h>
 #include <folly/Singleton.h>
 
-#ifdef __cplusplus
 extern "C" {
-#endif
 
 SingletonVault_t *SingletonVault_singleton() {
   return folly::SingletonVault::singleton();
@@ -37,6 +35,4 @@ void SingletonVault_reenableInstances(SingletonVault_t *vault) {
   ((folly::SingletonVault*) vault)->reenableInstances();
 }
 
-#ifdef __cplusplus
 } // extern "C"
-#endif