tests: Fix VPATH build
    
    So test-hotkeys include virt-viewer-app.h which includes
    virt-viewer-window.h which includes virt-viewer-display.h which
    in turn wants to include virt-viewer-enums.h. But, the enums
    header file is generated at build time into builddir not srcdir.
    Therefore it may happen if the two are distinct that compiler
    fails to find the enums file:
    
    In file included from ../../src/virt-viewer-window.h:29:0,
                     from ../../src/virt-viewer-app.h:28,
                     from ../../tests/test-hotkeys.c:27:
    ../../src/virt-viewer-display.h:28:31: fatal error: virt-viewer-enums.h: No such file or directory
     #include "virt-viewer-enums.h"
                                   ^
    compilation terminated.
    
    The fix is to include builddir into paths where header files are
    looked for.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Acked-by: Fabiano Fidêncio <fidencio@redhat.com>