KVM: selftests: Use proper function prototypes in probing code

Make the main() functions in the probing code proper prototypes so that
compiling the probing code with more strict flags won't generate false
negatives.

  <stdin>:1:5: error: function declaration isn’t a prototype [-Werror=strict-prototypes]
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20221213001653.3852042-8-seanjc@google.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/Makefile

@@ -194,11 +194,11 @@ CFLAGS += -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99 \
 	-I$(<D) -Iinclude/$(ARCH_DIR) -I ../rseq -I.. $(EXTRA_CFLAGS) \
 	$(KHDR_INCLUDES)
 
-no-pie-option := $(call try-run, echo 'int main() { return 0; }' | \
+no-pie-option := $(call try-run, echo 'int main(void) { return 0; }' | \
         $(CC) -Werror -no-pie -x c - -o "$$TMP", -no-pie)
 
 # On s390, build the testcases KVM-enabled
-pgste-option = $(call try-run, echo 'int main() { return 0; }' | \
+pgste-option = $(call try-run, echo 'int main(void) { return 0; }' | \
 	$(CC) -Werror -Wl$(comma)--s390-pgste -x c - -o "$$TMP",-Wl$(comma)--s390-pgste)
 
 LDLIBS += -ldl