fix(encrypt-fw): put build_msg under LOG_LEVEL flag

In tools directory, contrary to other parts of TF-A code,
LOG_LEVEL_NOTICE is 20, and LOG_LEVEL_ERROR is 10. If LOG_LEVEL is
set to 10, which is the case if BUILD_INFO=0, then we can have this
compilation warning:
src/main.c:29:19: warning: ‘build_msg’ defined but not used
 [-Wunused-const-variable=]
   29 | static const char build_msg[] = "Built : " __TIME__ ",
 " __DATE__;
      |                   ^~~~~~~~~

Avoid that by putting it under '#if LOG_LEVEL >= LOG_LEVEL_NOTICE'.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ic724610d6df811fc889775dbd361087e0958d31e
diff --git a/tools/encrypt_fw/src/main.c b/tools/encrypt_fw/src/main.c
index 6e43e73..b6246f6 100644
--- a/tools/encrypt_fw/src/main.c
+++ b/tools/encrypt_fw/src/main.c
@@ -26,7 +26,9 @@
 
 /* Global options */
 
+#if LOG_LEVEL >= LOG_LEVEL_NOTICE
 static const char build_msg[] = "Built : " __TIME__ ", " __DATE__;
+#endif
 
 static char *key_algs_str[] = {
 	[KEY_ALG_GCM] = "gcm",