Factor out time keeping code into its own module: mono_time.c.

It turns out, `unix_time` is also monotonic, and is used as such, so I've
renamed the new functions to `mono_time_*`.

2018-07-08:
```
00:01 <@irungentoo> the idea used to be that the unix_time() function
  could go backward in time but I think I might have started using it like
  if it could not after I changed it so that it would never go back in time
```

1 files changed, 1 insertions, 0 deletions

diff --git a/toxav/groupav.c b/toxav/groupav.c
index 8936801a..15fdd534 100644
--- a/toxav/groupav.c
+++ b/toxav/groupav.c
@@ -27,6 +27,7 @@
 #include <string.h>
 
 #include "../toxcore/logger.h"
+#include "../toxcore/mono_time.h"
 #include "../toxcore/util.h"
 
 #define GROUP_JBUF_SIZE 6