ipmid: fru-reader: Use timegm instead of mktime

timegm is the inverse of gmtime. Currently, we are missing UTC time with
localtime when converting between timestamp and time string. This is not
an issue when the localtime is the same as UTC.

This issue is detected when we change the timezone to PST/PDT with
https://gerrit.openbmc.org/c/openbmc/openbmc/+/58293
and the FRU EEPROM time does not match the expected Mfg Date anymore.

Required:
https://gerrit.openbmc.org/c/openbmc/ipmi-fru-parser/+/58446

Tested:
Fru EEPROM Mfg Time now is in the same (after timezone conversion)
between the raw data and FRU output.

Change-Id: I4c636b0044e1b2a6dd2c078b62f65665d4b1108c
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/ipmi_fru_info_area.cpp b/ipmi_fru_info_area.cpp
index a970467..ec3f80d 100644
--- a/ipmi_fru_info_area.cpp
+++ b/ipmi_fru_info_area.cpp
@@ -216,7 +216,7 @@
         }
     }
 
-    return std::mktime(&time);
+    return timegm(&time);
 }
 
 /**