plugins: Remove $(RSRT_LIBS) from imuxsock's LIBADD

imuxsock.c plugin uses some functions from sd-daemon.h that are
compiled as part of librsyslog.a. Including $(RSRT_LIB) to the LIBADD
variable on these plugins means that we are linking librsyslog.a
statically into that plugin. Nevertheless, the rsyslogd binary
already includes most of the librsyslog.a and exposes its symbols.
Because of this, we can let those functions undefined in the plugin
.so file and let dlopen() link those undefined functions against the
global symbols from rsyslogd.

This patch removes the static library from imuxsock.so and ensures
that all the objects from the librsyslog.a are included on rsyslogd.