Fixed Build fail issue when plugin-pcap build is diabled

Fixed Build fail issue when plugin-pcap build is diabled

Description:
This patch resolves the error which comes when plugin-pcap build is disabled.

Below error comes without this patch when plugin-pcap is diabled:-
=================================================================
[   20s] In file included from /home/abuild/rpmbuild/BUILD/stc-manager-0.0.89/include/stc-manager-plugin-pcap.h:22:0,
[   20s]                  from /home/abuild/rpmbuild/BUILD/stc-manager-0.0.89/src/stc-manager-plugin-pcap.c:20:
[   20s] /home/abuild/rpmbuild/BUILD/stc-manager-0.0.89/plugin/pcap/include/stc-plugin-pcap.h:28:18: fatal error: pcap.h: No such file or directory
[   20s]  #include <pcap.h>
[   20s]                   ^
[   20s] compilation terminated.
[   20s] src/CMakeFiles/stc-manager.dir/build.make:209: recipe for target 'src/CMakeFiles/stc-manager.dir/stc-manager-plugin-pcap.c.o' failed
[   20s] make[2]: *** [src/CMakeFiles/stc-manager.dir/stc-manager-plugin-pcap.c.o] Error 1
[   20s] make[2]: *** Waiting for unfinished jobs....
[   20s] In file included from /home/abuild/rpmbuild/BUILD/stc-manager-0.0.89/include/stc-manager-plugin-pcap.h:22:0,
[   20s]                  from /home/abuild/rpmbuild/BUILD/stc-manager-0.0.89/src/stc-pcap.c:19:
[   20s] /home/abuild/rpmbuild/BUILD/stc-manager-0.0.89/plugin/pcap/include/stc-plugin-pcap.h:28:18: fatal error: pcap.h: No such file or directory
[   20s]  #include <pcap.h>
[   20s]                   ^
[   20s] compilation terminated.
[   20s] src/CMakeFiles/stc-manager.dir/build.make:329: recipe for target 'src/CMakeFiles/stc-manager.dir/stc-pcap.c.o' failed
[   20s] make[2]: *** [src/CMakeFiles/stc-manager.dir/stc-pcap.c.o] Error 1
[   20s] In file included from /home/abuild/rpmbuild/BUILD/stc-manager-0.0.89/include/stc-manager-plugin-pcap.h:22:0,
[   20s]                  from /home/abuild/rpmbuild/BUILD/stc-manager-0.0.89/src/stc-manager.c:34:
[   20s] /home/abuild/rpmbuild/BUILD/stc-manager-0.0.89/plugin/pcap/include/stc-plugin-pcap.h:28:18: fatal error: pcap.h: No such file or directory
[   20s]  #include <pcap.h>
[   20s]                   ^
[   20s] compilation terminated.
=================================================================

Change-Id: I03c007d0adf19361304cc24acc85e4c5cd9d329d
Signed-off-by: Nishant Chaprana <n.chaprana@samsung.com>