build(encrypt-fw): don't generate `build_msg.c`

This change avoids generating a build message source file on the shell,
instead using the `__DATE__` and `__TIME__` macros directly.

Change-Id: Ida537d4c3e550f2fbbd977472ed6573491d17c23
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/tools/encrypt_fw/src/main.c b/tools/encrypt_fw/src/main.c
index 39b7af7..6e43e73 100644
--- a/tools/encrypt_fw/src/main.c
+++ b/tools/encrypt_fw/src/main.c
@@ -1,4 +1,5 @@
 /*
+ * Copyright (c) 2024, Arm Limited and Contributors. All rights reserved.
  * Copyright (c) 2019, Linaro Limited. All rights reserved.
  * Author: Sumit Garg <sumit.garg@linaro.org>
  *
@@ -25,8 +26,7 @@
 
 /* Global options */
 
-/* Info messages created in the Makefile */
-extern const char build_msg[];
+static const char build_msg[] = "Built : " __TIME__ ", " __DATE__;
 
 static char *key_algs_str[] = {
 	[KEY_ALG_GCM] = "gcm",