selftests: kvm: fix build with glibc >= 2.30

Glibc-2.30 gained gettid() wrapper, selftests fail to compile:

lib/assert.c:58:14: error: static declaration of ‘gettid’ follows non-static declaration
   58 | static pid_t gettid(void)
      |              ^~~~~~
In file included from /usr/include/unistd.h:1170,
                 from include/test_util.h:18,
                 from lib/assert.c:10:
/usr/include/bits/unistd_ext.h:34:16: note: previous declaration of ‘gettid’ was here
   34 | extern __pid_t gettid (void) __THROW;
      |                ^~~~~~
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/lib/assert.c

@@ -55,7 +55,7 @@ static void test_dump_stack(void)
 #pragma GCC diagnostic pop
 }
 
-static pid_t gettid(void)
+static pid_t _gettid(void)
 {
 	return syscall(SYS_gettid);
 }
@@ -72,7 +72,7 @@ test_assert(bool exp, const char *exp_str,
 		fprintf(stderr, "==== Test Assertion Failure ====\n"
 			"  %s:%u: %s\n"
 			"  pid=%d tid=%d - %s\n",
-			file, line, exp_str, getpid(), gettid(),
+			file, line, exp_str, getpid(), _gettid(),
 			strerror(errno));
 		test_dump_stack();
 		if (fmt) {