shairport-sync: bump to version 3.1.3

Fixes a runtime startup issue on SysV init systems:
```
# shairport-sync -d
Daemon returned 2 as return value.
```

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/shairport-sync/shairport-sync.hash

@@ -1,2 +1,2 @@
 # Locally calculated
-sha256  8c13f7ebbd417e8cab07ea9f74392ced0f54315d8697d4513580f472859a9c65  shairport-sync-3.1.2.tar.gz
+sha256  dd0484d7e8ee7631aee78c78b3762abbdba7ec3f2ee8cd6c1e361544c1414da3  shairport-sync-3.1.3.tar.gz

package/shairport-sync/shairport-sync.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-SHAIRPORT_SYNC_VERSION = 3.1.2
+SHAIRPORT_SYNC_VERSION = 3.1.3
 SHAIRPORT_SYNC_SITE = $(call github,mikebrady,shairport-sync,$(SHAIRPORT_SYNC_VERSION))
 
 SHAIRPORT_SYNC_LICENSE = MIT, BSD-3-Clause