bridge-utils: fix BUILD so that bridge-utils compiles.

If it uses the Linux headers from /usr/src/linux, it fails to compile
properly.  Using the Linux headers in /usr/include, it works.

Hat tip to the Gentoo guys for figuring that one out.
Merge pull request #225 from dagbrown/bridge-utils-build-fix

bridge-utils: fix BUILD so that bridge-utils compiles.