Fixed undefined references from libvpx.so in toxav_basic_test

Patch fixes the following link problem:
/usr/bin/ld: ../auto_tests/toxav_basic_test-toxav_basic_test.o: undefined reference to symbol 'vpx_img_alloc'
//usr/lib/x86_64-linux-gnu/libvpx.so.1: error adding symbols: DSO missing from command line