package/ninja: fix build for cmake 3.10

package/ninja: fix build for cmake 3.10

If the host cmake is 3.10, the configuration step produces
the following error:

CMake Error at CMakeLists.txt:87 (target_link_libraries):
Target "libninja" of type OBJECT_LIBRARY may not be linked into another
target. One may link only to STATIC or SHARED libraries, or to executables
with the ENABLE_EXPORTS property set.

This patch fixes CMakeLists.txt to use the object library as it was intended
in cmake 3.10.

Fixes:
https://bugs.busybox.net/show_bug.cgi?id=12546

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Tested-by: Damian Tometzki <dti@familie-tometzki.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>