s/IMPL_HEADER/IMPL_INTERFACE/g

More appropriate name for this enum constant now that they are defined
in the interface source files.

Test: hidl_test
Change-Id: Ice531adb797c3213a2da18421b7e97fcfc888da3
diff --git a/generateCpp.cpp b/generateCpp.cpp
index ce82edc..178b705 100644
--- a/generateCpp.cpp
+++ b/generateCpp.cpp
@@ -1900,7 +1900,7 @@
         method->generateCppSignature(out, iface->localName());
         if (reserved) {
             out.block([&]() {
-                method->cppImpl(IMPL_HEADER, out);
+                method->cppImpl(IMPL_INTERFACE, out);
             }).endl();
         }