checksyscalls: fix "here document" handling

"echo" doesn't read from stdin, therefore the checksyscalls script didn't
warn about not implemented system calls anymore since 29dc54c6
("checksyscalls: Use arch/x86/syscalls/syscall_32.tbl as source").

Use "cat" instead of "echo" which handles this correctly.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Michal Marek <mmarek@suse.cz>
Cc: H. Peter Anvin <hpa@linux.intel.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>