dropwatch: adjust LDFLAGS

Add more library links into LDFLAGS.
-lncurses because readline uses it (missing _tputs and others).
-lpthread for missing _pthread_rwlock_unlock and others.
-lm for missing _rint.
Fixes:
http://autobuild.buildroot.net/results/6b1/6b1f4f2167d8aca0d73dc0e66fd2e1704c821c61/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>