Fix localtime() calls for gpgme unsigned timestamps.

gpgme subkey/signature timestamps are unsigned long while localtime()
expects const time_t*; this fails to compile with GCC 16 / -std=c++14.

--- a/s.cpp
+++ b/s.cpp
@@ -265,7 +265,8 @@
 			/* determine subkeys per year */
 			if (subkeys -> timestamp > 0)
 			{
-				struct tm *sts = localtime(&subkeys -> timestamp);
+				time_t ts = subkeys -> timestamp;
+				struct tm *sts = localtime(&ts);
 
 				key_per_year.addvalue(sts -> tm_year + 1900, 1);
 				key_per_month.addvalue(sts -> tm_mon + 1, 1);
@@ -401,7 +402,8 @@
 				/* determine sigs per year/month/day */
 				if (sigs -> timestamp > 0)
 				{
-					struct tm *sts = localtime(&sigs -> timestamp);
+					time_t ts = sigs -> timestamp;
+					struct tm *sts = localtime(&ts);
 
 					sig_per_year.addvalue(sts -> tm_year + 1900, 1);
 					sig_per_month.addvalue(sts -> tm_mon + 1, 1);