powerpc: Export include/uapi/asm/eeh.h

This adds include/uapi/asm/eeh.h to kbuild so that the header
file will be exported automatically with below command. The
header file was added by commit ed3e81ff ("powerpc/eeh: Move PE
state constants around")

   make INSTALL_HDR_PATH=/tmp/headers \
        SRCARCH=powerpc headers_install

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>