[clang-tidy] add namespace qualifier NFC (#68579)

for daca972 #68134 adds a namespace as
we are not using llvm::StringRef yet

Author: metaflow

clang-tools-extra/clang-tidy/llvmlibc/NamespaceConstants.h

@@ -10,7 +10,7 @@
 
 namespace clang::tidy::llvm_libc {
 
-const static StringRef RequiredNamespaceStart = "__llvm_libc";
-const static StringRef RequiredNamespaceMacroName = "LIBC_NAMESPACE";
+const static llvm::StringRef RequiredNamespaceStart = "__llvm_libc";
+const static llvm::StringRef RequiredNamespaceMacroName = "LIBC_NAMESPACE";
 
 } // namespace clang::tidy::llvm_libc