net/dsa/tag_8021q.c: remove superfluous headers

tag_8021q.c hasn't use any macro or function declared in linux/if_bridge.h.
Thus, these files can be removed from tag_8021q.c safely without
affecting the compilation of the ./net/dsa module

Signed-off-by: Mianhan Liu <liumh1@shanghaitech.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>