time/Convert: add `pure` attributes

src/time/Convert.hxx

@@ -33,6 +33,8 @@
 #ifndef TIME_CONVERT_HXX
 #define TIME_CONVERT_HXX
 
+#include "util/Compiler.h"
+
 #include <chrono>
 
 /**
@@ -52,6 +54,7 @@ LocalTime(std::chrono::system_clock::time_point tp);
 /**
  * Convert a UTC-based "struct tm" to a UTC-based time point.
  */
+gcc_pure
 std::chrono::system_clock::time_point
 TimeGm(struct tm &tm);
 
@@ -60,6 +63,7 @@ TimeGm(struct tm &tm);
 /**
  * Convert a local "struct tm" to a UTC-based time point.
  */
+gcc_pure
 std::chrono::system_clock::time_point
 MakeTime(struct tm &tm);