Fix signedness warning in time_to_msec()

We have a signed integer here so let the return type
take in account that.

Found by Clang with -Weverything option.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/misc.h

@@ -51,7 +51,7 @@ struct Log : public std::ofstream {
 
 struct Time {
   void restart() { system_time(&t); }
-  uint64_t msec() const { return time_to_msec(t); }
+  int64_t msec() const { return time_to_msec(t); }
   int elapsed() const { return int(current_time().msec() - time_to_msec(t)); }
 
   static Time current_time() { Time t; t.restart(); return t; }

src/platform.h

@@ -48,7 +48,7 @@ typedef unsigned __int64 uint64_t;
 typedef timeval sys_time_t;
 
 inline void system_time(sys_time_t* t) { gettimeofday(t, NULL); }
-inline uint64_t time_to_msec(const sys_time_t& t) { return t.tv_sec * 1000LL + t.tv_usec / 1000; }
+inline int64_t time_to_msec(const sys_time_t& t) { return t.tv_sec * 1000LL + t.tv_usec / 1000; }
 
 #  include <pthread.h>
 typedef pthread_mutex_t Lock;
@@ -74,7 +74,7 @@ typedef void*(*pt_start_fn)(void*);
 typedef _timeb sys_time_t;
 
 inline void system_time(sys_time_t* t) { _ftime(t); }
-inline uint64_t time_to_msec(const sys_time_t& t) { return t.time * 1000LL + t.millitm; }
+inline int64_t time_to_msec(const sys_time_t& t) { return t.time * 1000LL + t.millitm; }
 
 #if !defined(NOMINMAX)
 #  define NOMINMAX // disable macros min() and max()