Fix PR17292 parse error of string auto-concatenation in printf statement.

Fix PR17292 parse error of string auto-concatenation in printf statement.

Use multi part string concatenation as the format string of printf
statement will cuase following parse error:

$ /usr/bin/stap -p1 testsuite/parseok/strconcat.stp
parse error: expected ','
        saw: string 'bar' at testsuite/parseok/strconcat.stp:6:19
     source:     printf("foo " "bar" "\n")
                               ^

1 parse error.
Pass 1: parse failed.  [man error::pass1]

This fix is suggested by fche.