Allow enabling of dummy error_strerror() to support some use-cases

Enable a dummy error function to make use of error_strerror() in
third party libraries easier.

Disable if you run into name conflicts and want to really remove the
error_strerror()
diff --git a/include/polarssl/config.h b/include/polarssl/config.h
index dac70e7..2ecb065 100644
--- a/include/polarssl/config.h
+++ b/include/polarssl/config.h
@@ -161,6 +161,17 @@
  */
 
 /**
+ * \def POLARSSL_ERROR_STRERROR_DUMMY
+ *
+ * Enable a dummy error function to make use of error_strerror() in
+ * third party libraries easier.
+ *
+ * Disable if you run into name conflicts and want to really remove the
+ * error_strerror()
+ */
+#define POLARSSL_ERROR_STRERROR_DUMMY
+
+/**
  * \def POLARSSL_GENPRIME
  *
  * Requires: POLARSSL_BIGNUM_C, POLARSSL_RSA_C