Add systemd to the PkgConfig file if it's needed by the lib

[mquinson]

Hello,

if I'm compiling an external project using the pkg config file, it fails with the following error on my machine:

FAILED: proxy 
: && /usr/bin/cc   CMakeFiles/proxy.dir/proxy.c.o -o proxy -L/opt/libwebsockets/lib -Wl,-rpath,/opt/libwebsockets/lib  -lm  -lwebsockets  -lcap && :
/usr/bin/ld: /opt/libwebsockets/lib/libwebsockets.so: undefined reference to `sd_is_socket_inet'
/usr/bin/ld: /opt/libwebsockets/lib/libwebsockets.so: undefined reference to `sd_listen_fds'
/usr/bin/ld: /opt/libwebsockets/lib/libwebsockets.so: undefined reference to `sd_is_socket_unix'
collect2: error: ld returned 1 exit status

This PR adds SystemD to the dependencies of the package if SystemD was detected on disk.

Thanks for this great project,
Mt