Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239
Change-Id: I5b4123bc6709641315120a191e36cc57541349b2
diff --git a/libc/bionic/reboot.cpp b/libc/bionic/reboot.cpp
index 9cf4411..b0a1f97 100644
--- a/libc/bionic/reboot.cpp
+++ b/libc/bionic/reboot.cpp
@@ -32,5 +32,5 @@
 extern "C" int __reboot(int, int, int, void*);
 
 int reboot(int mode) {
-  return __reboot(LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, mode, NULL);
+  return __reboot(LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, mode, nullptr);
 }