KVM: selftests: Add new CFLAGS to generate dependency files

KVM: selftests: Add new CFLAGS to generate dependency files

Add "-MD" in CFLAGS to generate dependency files. Currently, each
time a header file is updated in KVM selftest, we will have to run
"make clean && make" to rebuild the whole test suite. By adding new
compiling flags and dependent rules in Makefile, we do not need to
make clean && make each time a header file is updated.

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Link: https://lore.kernel.org/r/20230601080338.212942-1-yu.c.zhang@linux.intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>