Add 'MBEDTLS_DHM_ALT' #DEFINE to library/config.h

Add 'MBEDTLS_DHM_ALT' #DEFINE to library/config.h to support alternate DHM
diff --git a/library/dhm.c b/library/dhm.c
index 6f8f021..882b306 100644
--- a/library/dhm.c
+++ b/library/dhm.c
@@ -580,7 +580,8 @@
 }
 #endif /* MBEDTLS_FS_IO */
 #endif /* MBEDTLS_ASN1_PARSE_C */
-#endif/*MBEDTLS_DHM_ALT*/
+#endif /* MBEDTLS_DHM_ALT */
+
 #if defined(MBEDTLS_SELF_TEST)
 
 static const char mbedtls_test_dhm_params[] =