Don't include openssl/ec_key.h under extern "C".

Reportedly some combination of C++ modules and old clang gets upset.
That seems an inadvisable combination, but including headers under
extern "C" is rude, so fix it.

Change-Id: I12f873e1be41697b67f2b1145387a3c6fc769c28
Reviewed-on: https://boringssl-review.googlesource.com/c/33024
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

include/openssl/ec.h

@@ -348,13 +348,15 @@ OPENSSL_EXPORT size_t EC_get_builtin_curves(EC_builtin_curve *out_curves,
 // EC_POINT_clear_free calls |EC_POINT_free|.
 OPENSSL_EXPORT void EC_POINT_clear_free(EC_POINT *point);
 
-// Old code expects to get EC_KEY from ec.h.
-#include <openssl/ec_key.h>
-
 
 #if defined(__cplusplus)
 }  // extern C
+#endif
 
+// Old code expects to get EC_KEY from ec.h.
+#include <openssl/ec_key.h>
+
+#if defined(__cplusplus)
 extern "C++" {
 
 BSSL_NAMESPACE_BEGIN