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/md2.h b/include/mbedtls/md2.h
index 08e75b2..d60d0f8 100644
--- a/include/mbedtls/md2.h
+++ b/include/mbedtls/md2.h
@@ -282,6 +282,8 @@
 #undef MBEDTLS_DEPRECATED
 #endif /* !MBEDTLS_DEPRECATED_REMOVED */
 
+#if defined(MBEDTLS_SELF_TEST)
+
 /**
  * \brief          Checkup routine
  *
@@ -294,6 +296,8 @@
  */
 int mbedtls_md2_self_test( int verbose );
 
+#endif /* MBEDTLS_SELF_TEST */
+
 #ifdef __cplusplus
 }
 #endif