libc: Make tv_usec in timeval signed, as it should be

libs/libc/include/sys/types.h

@@ -25,4 +25,7 @@ typedef long int clock_t;
 /* Value representing time in seconds. */
 typedef long int time_t;
 
+/* Type representing signed time in microseconds. */
+typedef long int suseconds_t;
+
 #endif

libs/libc/include/time.h

@@ -25,7 +25,7 @@ struct tm
 struct timeval
 {
     time_t tv_sec;
-    unsigned long tv_usec;
+    suseconds_t tv_usec;
 };
 
 #define CLOCKS_PER_SEC 1000 // Number of clock_t per second.