utils: Deprecate is_time_legal()

It isn't used in any OpenBMC repositories depending on libpldm. Likely
it should have been internal, but it was exposed in a public header from
the beginning. Start the process of removing it from the public API.

Change-Id: I81ab0e07c832b93322d6a17287483719d435307c
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/src/utils.c b/src/utils.c
index 71ae500..b884cca 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -232,7 +232,7 @@
 	}
 }
 
-LIBPLDM_ABI_STABLE
+LIBPLDM_ABI_DEPRECATED
 bool is_time_legal(uint8_t seconds, uint8_t minutes, uint8_t hours, uint8_t day,
 		   uint8_t month, uint16_t year)
 {