Fix ODR issue in StrongPointer_test.cpp

StrongPointer_test.cpp's Foo was colliding with RefBase_test.cpp's
Foo.

Test: out/host/linux-x86/nativetest64/libutils_tests/libutils_tests
      from later CL
Change-Id: I2a4e956c88a07cec72d7ce734cf06c58134a4235
1 file changed