Fix Build warning C4068 for MSVC (#221)

- GCC pragma is not a valid pragma in MSVC, thus produces warning C4068:
unknown pragma 'GCC'.
- The workaround is to encapsulate the GCC pragma in if GNUC defined
block.

Signed-off-by: Avinal Kumar <avinal.xlvii@gmail.com>

src/electionguard/utils.hpp

@@ -38,10 +38,16 @@ namespace electionguard
     {
         auto now = system_clock::now();
         auto ticks = now.time_since_epoch();
+
+#if defined(__GNUC__)        
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wsign-conversion"
+#endif // __GNUC__
         return ticks.count() * system_clock::period::num / system_clock::period::den;
+#if defined(__GNUC__)
 #pragma GCC diagnostic pop
+#endif // __GNUC__
+
     }
 
     /// <Summary>