kbuild: silence Nothing to be done for 'all' message

This patch silences a Makefile.asm-generic message
by defining a dummy rule for all.

make -f /usr/src/git/scripts/Makefile.asm-generic \
            obj=arch/x86/include/generated/asm
make[1]: Nothing to be done for `all'.

Signed-off-by: Peter Foley <pefoley2@verizon.net>
Signed-off-by: Michal Marek <mmarek@suse.cz>