wireshark: fix wrong CFLAGS_FOR_BUILD assumption

Tested valid CFLAGS can't be pushed into CFLAGS_FOR_BUILD, they're
different compilers.

Fixes:
http://autobuild.buildroot.net/results/2b1/2b1a0654b649618422d65edf370b82476ba84491/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>