added 'make clean' fix

[keith4ever]

quick fix of "libQnnTFLiteDelegate.so missing" issue, caused by mixed builds of QNN & GPU delegate.

[atiorh]

Fixes #10

[bpkeene]

noted offline:

• need to copy the qnn libs & skel files to libs/android, currently just in libs/
• need to mkdir -p libs/android and libs/x86;
• note that libs/ is under .gitignore
• ensure conformance between docker env creation & associated build scripts relying on docker configuration for relevant assets.