gdbhooks: regex syntax error

Recent python complains about this pattern with
  SyntaxWarning: invalid escape sequence '\s'
because \s in a regular string just means 's'; for it to mean whitespace,
you need \\ or for the pattern to be a raw string.

Curiously, break-on-pass completion works for me either with or without this
change, but at least this avoids the warning.

gcc/ChangeLog:

	* gdbhooks.py: Fix regex syntax.