LibC: Use correct macro to disable assert()

The C standard doesn't say anything about DEBUG, just NDEBUG :^)

Userland/Libraries/LibC/assert.cpp

@@ -16,7 +16,7 @@
 extern "C" {
 
 extern bool __stdio_is_initialized;
-#ifdef DEBUG
+#ifndef NDEBUG
 void __assertion_failed(const char* msg)
 {
     dbgln("USERSPACE({}) ASSERTION FAILED: {}", getpid(), msg);

Userland/Libraries/LibC/assert.h

@@ -10,7 +10,7 @@
 
 __BEGIN_DECLS
 
-#ifdef DEBUG
+#ifndef NDEBUG
 __attribute__((noreturn)) void __assertion_failed(const char* msg);
 #    define __stringify_helper(x) #    x
 #    define __stringify(x) __stringify_helper(x)