KVM: sefltests: Explicitly include ucall_common.h in mmu_stress_test.c

Explicitly include ucall_common.h in the MMU stress test, as unlike arm64
and x86-64, RISC-V doesn't include ucall_common.h in its processor.h, i.e.
this will allow enabling the test on RISC-V.

Reported-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20241128005547.4077116-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/mmu_stress_test.c

@@ -15,6 +15,7 @@
 #include "test_util.h"
 #include "guest_modes.h"
 #include "processor.h"
+#include "ucall_common.h"
 
 static void guest_code(uint64_t start_gpa, uint64_t end_gpa, uint64_t stride)
 {