Q3BSP: Fix build with clang libc++

code/Q3BSPZipArchive.cpp

@@ -43,6 +43,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include "Q3BSPZipArchive.h"
 #include <cassert>
+#include <cstdlib>
 #include <assimp/ai_assert.h>
 
 namespace Assimp {