gcc-plugins: Detail c-common.h location for GCC 4.6

The c-common.h file moved in stock gcc 4.7, not gcc 4.6. However, most
people building plugins with gcc 4.6 are using the Debian or Ubuntu
version, which includes a patch to move the headers to the 4.7 location.
In case anyone trips over this with a stock gcc 4.6, add a pointer to the
patch used by Debian/Ubuntu.

Signed-off-by: Kees Cook <keescook@chromium.org>