Fix linking on newer SmartOS versions (#7)

Newer SmartOS versions require the 'xnet' library to be explicitly
declared to the linker. While this might not be needed on plain Solaris
this is good form non the less (since bcrypt uses symbols from the net
library).

rebar.config

@@ -7,7 +7,7 @@
 {port_env, [
   {"DRV_LDFLAGS","-shared $ERL_LDFLAGS -lpthread"},
   {"darwin", "DRV_LDFLAGS", "-bundle -flat_namespace -undefined suppress $ERL_LDFLAGS -lpthread"},
-  {"solaris", "ERL_LDFLAGS", "-lssp -lnsl $ERL_LDFLAGS"},
+  {"solaris", "ERL_LDFLAGS", "-lxnet -lssp -lnsl $ERL_LDFLAGS"},
   {"DRV_CFLAGS","-Ic_src -Wall -fPIC $ERL_CFLAGS"},
   {"LDFLAGS", "$LDFLAGS -lpthread"}
 ]}.