lisp_helper: fix non-literal in printf().

When compiling with gcc-4.6.3, the following error occurs:

lisp_helper.c:110:3: error: format not a string literal and no format
arguments [-Werror=format-security]

This patch eliminates the warning.
Merge pull request #1 from tcort/compile_fix

lisp_helper: fix non-literal in printf().