Fixes cppcheck warnings:
sys/net/network_layer/ng_ndp/ng_ndp.c:548: style (variableScope): The scope of the variable 'l2src' can be reduced.
sys/net/network_layer/ng_ndp/ng_ndp.c:549: style (variableScope): The scope of the variable 'l2src_len' can be reduced.
sys/net/network_layer/ng_ndp/ng_ndp.c:608: style (variableScope): The scope of the variable 'l2src' can be reduced.
sys/net/network_layer/ng_ndp/ng_ndp.c:609: style (variableScope): The scope of the variable 'l2src_len' can be reduced.