test_libvpx: fix negative CPU filter

negative patterns begin with _one_ '-', not one before each pattern.

https://code.google.com/p/googletest/wiki/V1_7_AdvancedGuide#Running_a_Subset_of_the_Tests

Change-Id: I6fd527b31bc2571dfdaa453cbbb57c77b3409563
1 file changed