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/rsa.h b/include/mbedtls/rsa.h
index 19eb2ee..71b5c2e 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -1116,6 +1116,8 @@
  */
 void mbedtls_rsa_free( mbedtls_rsa_context *ctx );
 
+#if defined(MBEDTLS_SELF_TEST)
+
 /**
  * \brief          The RSA checkup routine.
  *
@@ -1124,6 +1126,8 @@
  */
 int mbedtls_rsa_self_test( int verbose );
 
+#endif /* MBEDTLS_SELF_TEST */
+
 #ifdef __cplusplus
 }
 #endif