[ENB] Fix enabling of MAC pcap file

when using

[pcap]
enable = true
filename = /tmp/enb.pcap

in enb.conf, there is no pcap file created.

The problem is somewhere in the way how arguments are handled.
pcap.enable is properly parsed into args.pcap.  However, later on,
lte_stack->init(args.stack, rrc_cfg, lte_phy.get()) only passes
args.stack down the road, not args.pcap.  enb_stack_lte::init() then
basically uses args.stack.pcap and not args.pcap, and the latter appears
always false.

Let's remove pcap_args_t from all_args and only use the instance in
stack_args_t.

Closes: #359

srsenb/hdr/enb.h

@@ -101,7 +101,6 @@ struct all_args_t {
   enb_args_t        enb;
   enb_files_t       enb_files;
   srslte::rf_args_t rf;
-  pcap_args_t       pcap;
   log_args_t        log;
   gui_args_t        gui;
   general_args_t    general;

srsenb/src/main.cc

@@ -97,8 +97,8 @@ void parse_args(all_args_t *args, int argc, char* argv[]) {
     ("rf.time_adv_nsamples", bpo::value<string>(&args->rf.time_adv_nsamples)->default_value("auto"), "Transmission time advance")
     ("rf.burst_preamble_us", bpo::value<string>(&args->rf.burst_preamble)->default_value("auto"),    "Transmission time advance")
 
-    ("pcap.enable",       bpo::value<bool>(&args->pcap.enable)->default_value(false),         "Enable MAC packet captures for wireshark")
-    ("pcap.filename",     bpo::value<string>(&args->pcap.filename)->default_value("ue.pcap"), "MAC layer capture filename")
+    ("pcap.enable",       bpo::value<bool>(&args->stack.pcap.enable)->default_value(false),         "Enable MAC packet captures for wireshark")
+    ("pcap.filename",     bpo::value<string>(&args->stack.pcap.filename)->default_value("ue.pcap"), "MAC layer capture filename")
 
     ("gui.enable",        bpo::value<bool>(&args->gui.enable)->default_value(false),          "Enable GUI plots")