add darwin specific uptime variant -- not implemented and never will

uptime_darwin.go

@@ -0,0 +1,12 @@
+//go:build darwin
+
+package themis
+
+import (
+	"time"
+)
+
+// Uptime returns the time elapsed since the start of the current process ID.
+func Uptime() (time.Duration, error) {
+	panic("not implemented")
+}

uptime_linux.go

@@ -1,3 +1,5 @@
+//go:build linux
+
 package themis
 
 import (