Tests/NinjaPrivateDeps: fix error message for Build-PrivateFileSet-check

The file *should* exist, so word the error message as such.

Tests/RunCMake/NinjaPrivateDeps/CMP0154-NEW-Build-PrivateFileSet-check.cmake

@@ -1,3 +1,3 @@
 if (NOT EXISTS ${RunCMake_TEST_BINARY_DIR}/private.h)
-  set(RunCMake_TEST_FAILED "Private header generated before compilation.")
+  set(RunCMake_TEST_FAILED "Private header should be generated for target compilation")
 endif()