libpfm4: needs thread support

Fixes:

  http://autobuild.buildroot.net/results/73d736dd3c8a70358ef4b19a63dda46178cf8bf1/



Note that the propagation of the thread dependency to the oprofile
package is a little bit non standard, because oprofile selects libpfm4
only on the PowerPC architecture. So we ensure the thread dependency
is only enforced on PowerPC, and a separate comment is displayed when
thread support is not available, but the PowerPC architecture is used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>