package/ser2net: fix gensio detection with openssl enabled and static

Add openssl linker flags via LIBS to fix configure gensio
library detection.

Fixes:

  http://autobuild.buildroot.net/results/66e0d3e0a2a8dc5a62c267d16a53216f0f2ce8dd



  checking gensio/gensio.h usability... yes
  checking gensio/gensio.h presence... yes
  checking for gensio/gensio.h... yes
  checking for str_to_gensio in -lgensio... no
  configure: error: libgensio won't link, please install gensio dev package

The build/ser2net-4.1.1/config.log files states:

  .../arm-buildroot-linux-uclibcgnueabi/bin/ld: .../host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libgensio.a(gensio_filter_ssl.o): in function `gensio_do_ssl_init':
  gensio_filter_ssl.c:(.text+0x34): undefined reference to `OPENSSL_init_ssl'

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>