Revert "hwcomposer: fix parallel builds"

This reverts commit 151fe1dfad8f6b9ce75b5b81dea657b78c013c58.
The library *is* actually used by more than test_hwcomposer,
for example by Carstens' qt hwcomposer platform plugin.
Merge pull request #96 from groleo/tz

Revert "hwcomposer: fix parallel builds"