drivers/bios_emulator: Fix compile error in .depend not being generated

drivers/bios_emulator: Fix compile error in .depend not being generated

make -C drivers/bios_emulator/
make[2]: Entering directory
`drivers/bios_emulator'
In file included from atibios.c:49:
biosemui.h:47:21: error: biosemu.h: No such file or directory
...
x86emu/decode.c:40:28: error: x86emu/x86emui.h: No such file or directory
...

Due to lack of proper CPPFLAGS being passed to .depend generation rule

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>