gen-strtab.awk: Work around parse issue on HP-UX 11.

HP-UX 11 awk seems to have some issues parsing ! in expressions.
For example:

  % awk 'BEGIN { print 1 + !0 }'
  syntax error The source line is 1.
  The error context is
		BEGIN { print 1 + >>>  ! <<< 0 }
  awk: The statement cannot be correctly parsed.
  The source line is 1.

Adding parentheses appers sufficient to avoid the problem:

  % awk 'BEGIN { print 1 + (!0) }'
  2