Fix compilation error on gcc

https://crrev.com/c/3471558 is causing the following compilation
error on gcc:
```
error: suggest explicit braces to avoid ambiguous 'else'
```

Bug: chromium:1298417
Change-Id: I84a34603664c5ee148cc9ea282c0f8c53319b6d8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3472403Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#79154}

test/unittests/heap/cppgc/page-memory-unittest.cc

@@ -298,8 +298,9 @@ TEST(PageBackendTest, LookupNormal) {
   PageBackend backend(allocator, oom_handler);
   constexpr size_t kBucket = 0;
   Address writeable_base = backend.AllocateNormalPageMemory(kBucket);
-  if (kGuardPageSize)
+  if (kGuardPageSize) {
     EXPECT_EQ(nullptr, backend.Lookup(writeable_base - kGuardPageSize));
+  }
   EXPECT_EQ(nullptr, backend.Lookup(writeable_base - 1));
   EXPECT_EQ(writeable_base, backend.Lookup(writeable_base));
   EXPECT_EQ(writeable_base, backend.Lookup(writeable_base + kPageSize -
@@ -318,8 +319,9 @@ TEST(PageBackendTest, LookupLarge) {
   PageBackend backend(allocator, oom_handler);
   constexpr size_t kSize = 7934;
   Address writeable_base = backend.AllocateLargePageMemory(kSize);
-  if (kGuardPageSize)
+  if (kGuardPageSize) {
     EXPECT_EQ(nullptr, backend.Lookup(writeable_base - kGuardPageSize));
+  }
   EXPECT_EQ(nullptr, backend.Lookup(writeable_base - 1));
   EXPECT_EQ(writeable_base, backend.Lookup(writeable_base));
   EXPECT_EQ(writeable_base, backend.Lookup(writeable_base + kSize - 1));