libpthsem: needs mmu

The need can be patched out by disabling the tests, however the only
package that uses libpthsem (bcusdk) needs a fork-enabled libpthsem so
there's not much point. Fixes:

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>