gcc: add build directory to include directories

gcc: add build directory to include directories

Add build directory to include directories by -I${B}/include which will
be searched before standard system include directories.

Both libunwind and gcc supply unwind.h, once gcc use libunwind's unwind.h,
the compiling will fail.

This patch is generally not applicable to the upstream as they do not
use libunwind.

(From OE-Core rev: 2b47bce78536a800205b2385bba69038351545e5)

Signed-off-by: Roy.Li <rongqing.li@windriver.com>
Signed-off-by: Jeff Polk <jeff.polk@windriver.com>
Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>