sparse: Support recent distributions

sparse: Support recent distributions

sparse support seems to be broken on some recent Linux distributions.
For example, ubuntu 12.04 with Linux 3.5 kernel, and Debian latest test
distribution, running Linux 3.2 kernel.

On both systems that sparse was broken, It was not able find the header files
in the  default system include directories.  GCC finds them by default.

This patch adds the required GCC default search path when running sparse.

Tested on:

Ubuntu 12.04 - w/ linux 3.5 kernel
Debian-6 March test distribution - w/ linux 3.2 kernel

Signed-off-by: Andy Zhou <azhou@nicira.com>
[blp@nicira.com also tested on Debian squeeze and wheezy (testing)]
Signed-off-by: Ben Pfaff <blp@nicira.com>