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/ChangeLog b/ChangeLog
index 348864c..4ea209d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -13,6 +13,10 @@
      Therefore, mbedtls_platform_zeroize() is moved to the platform module to
      facilitate testing and maintenance.
 
+Bugfix
+   * Add `MBEDTLS_SELF_TEST` for the mbedtls_self_test functions
+     in the header files, which missed the precompilation check. #971
+
 = mbed TLS 2.9.0 branch released 2018-04-30
 
 Security