trace: Use strncmp() to match function names

trace: Use strncmp() to match function names

The functions specified by the user might be longer than the function in
the backtrace, potentially overflowing the memcmp(). In practice, it
should not be a relevant out-of-memory read. However, we can use
strncmp() instead.

Note that, as before, this is only a prefix match. If a function name is
longer in the backtrace it will still match.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>