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/dhm.h b/include/mbedtls/dhm.h
index 75317a8..2f5b5a0 100644
--- a/include/mbedtls/dhm.h
+++ b/include/mbedtls/dhm.h
@@ -295,6 +295,8 @@
 #endif /* MBEDTLS_FS_IO */
 #endif /* MBEDTLS_ASN1_PARSE_C */
 
+#if defined(MBEDTLS_SELF_TEST)
+
 /**
  * \brief          The DMH checkup routine.
  *
@@ -303,6 +305,7 @@
  */
 int mbedtls_dhm_self_test( int verbose );
 
+#endif /* MBEDTLS_SELF_TEST */
 #ifdef __cplusplus
 }
 #endif