Flip the value of the incompatible flag --incompatible_validate_top_level_header_inclusions.

Work towards #10047.

RELNOTES: None.
PiperOrigin-RevId: 298577249
diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/CppOptions.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/CppOptions.java
index 55ca0ce..4047d17 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/CppOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/CppOptions.java
@@ -790,7 +790,7 @@
 
   @Option(
       name = "incompatible_validate_top_level_header_inclusions",
-      defaultValue = "false",
+      defaultValue = "true",
       documentationCategory = OptionDocumentationCategory.INPUT_STRICTNESS,
       effectTags = {OptionEffectTag.LOADING_AND_ANALYSIS},
       metadataTags = {