scripts/bloat-o-meter: use .startswith rather than fragile slicing

str.startswith has existed since at least Python 2.0, in 2000; use it
rather than a fragile comparison against an initial slice of a string,
which requires hard-coding the length of the string to compare against.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>