Add `MBEDTLS_SELF_TEST` check in header files

Add the `MBEDTLS_SELF_TEST` precompilation surrounding the self test functions,
which were missing this check in the header files. ( most of the header files were missing this check).
Addresses issue #971
diff --git a/include/mbedtls/aes.h b/include/mbedtls/aes.h
index e0fc238..2aef993 100644
--- a/include/mbedtls/aes.h
+++ b/include/mbedtls/aes.h
@@ -400,6 +400,8 @@
 #undef MBEDTLS_DEPRECATED
 #endif /* !MBEDTLS_DEPRECATED_REMOVED */
 
+
+#if defined(MBEDTLS_SELF_TEST)
 /**
  * \brief          Checkup routine.
  *
@@ -408,6 +410,8 @@
  */
 int mbedtls_aes_self_test( int verbose );
 
+#endif /* MBEDTLS_SELF_TEST */
+
 #ifdef __cplusplus
 }
 #endif