Fix parallel build error (make -jX)

Add makefile dependency to fix the following:

$ make -j3 CROSS_COMPILE=arm-linux-gnueabihf-
...
cp out/libteec/libteec.so* out/export/lib
cp: cannot stat ‘out/libteec/libteec.so*’: No such file or directory
make: *** [copy_export] Error 1