Fix misplaced extern "C" affecting MBEDTLS_ARIA_ALT

Reported via Mbed OS:
https://github.com/ARMmbed/mbed-os/issues/14694

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/aria.h b/include/mbedtls/aria.h
index a4b27b3..6c73d10 100644
--- a/include/mbedtls/aria.h
+++ b/include/mbedtls/aria.h
@@ -61,14 +61,14 @@
 /* MBEDTLS_ERR_ARIA_HW_ACCEL_FAILED is deprecated and should not be used. */
 #define MBEDTLS_ERR_ARIA_HW_ACCEL_FAILED      -0x0058  /**< ARIA hardware accelerator failed. */
 
-#if !defined(MBEDTLS_ARIA_ALT)
-// Regular implementation
-//
-
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+#if !defined(MBEDTLS_ARIA_ALT)
+// Regular implementation
+//
+
 /**
  * \brief The ARIA context-type definition.
  */