selftests: kvm: Fix a compile error in selftests/kvm/rseq_test.c

The following warning appears when executing:
	make -C tools/testing/selftests/kvm

rseq_test.c: In function ‘main’:
rseq_test.c:237:33: warning: implicit declaration of function ‘gettid’; did you mean ‘getgid’? [-Wimplicit-function-declaration]
          (void *)(unsigned long)gettid());
                                 ^~~~~~
                                 getgid
/usr/bin/ld: /tmp/ccr5mMko.o: in function `main':
../kvm/tools/testing/selftests/kvm/rseq_test.c:237: undefined reference to `gettid'
collect2: error: ld returned 1 exit status
make: *** [../lib.mk:173: ../kvm/tools/testing/selftests/kvm/rseq_test] Error 1

Use the more compatible syscall(SYS_gettid) instead of gettid() to fix it.
More subsequent reuse may cause it to be wrapped in a lib file.
Signed-off-by: Jinrong Liang <cloudliang@tencent.com>
Message-Id: <20220802071240.84626-1-cloudliang@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/rseq_test.c

@@ -227,7 +227,7 @@ int main(int argc, char *argv[])
 	ucall_init(vm, NULL);
 
 	pthread_create(&migration_thread, NULL, migration_worker,
-		       (void *)(unsigned long)gettid());
+		       (void *)(unsigned long)syscall(SYS_gettid));
 
 	for (i = 0; !done; i++) {
 		vcpu_run(vcpu);