fix building script issue.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1541 bbd45198-f89e-11dd-88c7-29a3b14d5316

components/net/lwip/SConscript

@@ -47,8 +47,6 @@ src/core/snmp/mib_structs.c
 src/core/snmp/msg_in.c
 src/core/snmp/msg_out.c
 """)
-if GetDepend(['RT_LWIP_PPP']):
-    src += ppp_src
 
 ppp_src = Split("""
 src/netif/ppp/auth.c