LibC: Fix some incorrect printf usages

Author: https://github.com/ccapitalK
Commit: d780e2265d
Pull-request: https://github.com/SerenityOS/serenity/pull/4532
Reviewed-by: https://github.com/ADKaster

Userland/Tests/LibC/snprintf-correctness.cpp

@@ -32,6 +32,8 @@
 #include <ctype.h>
 #include <stdio.h>
 
+#pragma GCC diagnostic ignored "-Wformat-nonliteral"
+
 struct Testcase {
     const char* dest;
     size_t dest_n;