[server] bug fix to exclude pcap.h only in --enable-udp-server mode

This commit fixes issue #143 on github reported by Coacher. The previous
commit introduced a build time error for non UDP server mode as seen
here:

https://paste.kde.org/pkaxwobwr

server/pcap_capture.c

@@ -30,8 +30,8 @@
 */
 
 #if USE_LIBPCAP
-
-#include <pcap.h>
+  #include <pcap.h>
+#endif
 
 #include "fwknopd_common.h"
 #include "pcap_capture.h"
@@ -46,6 +46,8 @@
   #include <sys/wait.h>
 #endif
 
+#if USE_LIBPCAP
+
 /* The pcap capture routine.
 */
 int

server/process_packet.c

@@ -33,8 +33,8 @@
 */
 
 #if USE_LIBPCAP
-
-#include <pcap.h>
+  #include <pcap.h>
+#endif
 
 #include "fwknopd_common.h"
 #include "netinet_common.h"
@@ -43,6 +43,8 @@
 #include "utils.h"
 #include "log_msg.h"
 
+#if USE_LIBPCAP
+
 void
 process_packet(unsigned char *args, const struct pcap_pkthdr *packet_header,
     const unsigned char *packet)