clamav: fix bzip2 detection

clamav: fix bzip2 detection

Configure gets confused if the host has bzip2 development headers, so force
the results.

Fixes:
http://autobuild.buildroot.net/results/e73/e732d1bac8fe68fd8bba50e4e9d908be3d996c83/
http://autobuild.buildroot.net/results/1a4/1a46e53cf892534f1b3a16c249fa710485290b5a/
http://autobuild.buildroot.net/results/6d0/6d09379aaba0ccddddfee9e319b84687012fd5fc/
http://autobuild.buildroot.net/results/d23/d2310a2f265e7d22c025a61e064a3c29dc6213ef/

And many more.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>