simpleperf: don't add source code and disassembly for unknown functions.

Unknown functions are for samples without symbol info (Either because
the shared library can't be found, or the symbol table is missing).
So it doesn't make sence to add source code or disassembly for unknown
functions.

Bug: http://b/66914187
Test: run report_html.py manually.
Test: run test.py.

Change-Id: Ic1573792b6a522db61969a66dcb68c737d38749d
1 file changed