input: Move get_current_time_msec in from util

get_current_time_msec is only used in cursor.c, so we can move it in and
make it static. This is primarily intended to avoid a symbol collision
with wlroots, which we unfortunately do not have a good solution for
yet.

include/util.h

@@ -29,12 +29,6 @@ enum movement_unit parse_movement_unit(const char *unit);
 int parse_movement_amount(int argc, char **argv,
 		struct movement_amount *amount);
 
-/**
- * Get the current time, in milliseconds.
- */
-
-uint32_t get_current_time_msec(void);
-
 /**
  * Wrap i into the range [0, max]
  */