FS-10055: Fix gentls_cert script to use "@certsdir@"

Currently gentls_cert uses "@prefix@/conf/ssl". But FreeSWITCH's
configure script provides "@certsdir@", so let's use that instead.
Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>

scripts/gentls_cert.in

 #!/bin/sh
 
-CONFDIR=@prefix@/conf/ssl
+CONFDIR=@certsdir@
 DAYS=2190
 KEY_SIZE=2048
 export KEY_SIZE=${KEY_SIZE}