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/pkcs5.h b/include/mbedtls/pkcs5.h
index 9a3c9fd..fe56cbd 100644
--- a/include/mbedtls/pkcs5.h
+++ b/include/mbedtls/pkcs5.h
@@ -81,6 +81,8 @@
                        unsigned int iteration_count,
                        uint32_t key_length, unsigned char *output );
 
+#if defined(MBEDTLS_SELF_TEST)
+
 /**
  * \brief          Checkup routine
  *
@@ -88,6 +90,8 @@
  */
 int mbedtls_pkcs5_self_test( int verbose );
 
+#endif /* MBEDTLS_SELF_TEST */
+
 #ifdef __cplusplus
 }
 #endif