component/openvpn: link only with slapos libs

if libnsl was installed on the system, openvpn would link with it
so be proactive and always link with slapos libnsl.

component/openvpn/buildout.cfg

 [buildout]
 extends =
+  ../flex/buildout.cfg
+  ../libnsl/buildout.cfg
   ../lzo/buildout.cfg
   ../openssl/buildout.cfg
-  ../flex/buildout.cfg
   ../xz-utils/buildout.cfg
 
 parts =
@@ -23,5 +24,5 @@ environment =
   LZO_CFLAGS=-I${lzo:location}/include
   OPENSSL_LIBS=-L${openssl-1.1:location}/lib -lcrypto -lssl
   OPENSSL_CFLAGS=-I${openssl-1.1:location}/include
-  LDFLAGS =-Wl,-rpath=${lzo:location}/lib -Wl,-rpath=${flex:location}/lib -Wl,-rpath=${openssl-1.1:location}/lib
-  CPPFLAGS=-I${lzo:location}/include -I${flex:location}/include
+  LDFLAGS =-Wl,-rpath=${lzo:location}/lib -Wl,-rpath=${flex:location}/lib -Wl,-rpath=${openssl-1.1:location}/lib -L${libnsl:location}/lib -Wl,-rpath=${libnsl:location}/lib
+  CPPFLAGS=-I${lzo:location}/include -I${flex:location}/include -I${libnsl:location}/include