Turn on -Wunused and fix the mistakes it uncovers.

Change-Id: I023d2d8b547fbc21d4124bb7510d42b06a0dc501
diff --git a/libc/bionic/libc_init_static.cpp b/libc/bionic/libc_init_static.cpp
index e45422e..ab0b3a6 100644
--- a/libc/bionic/libc_init_static.cpp
+++ b/libc/bionic/libc_init_static.cpp
@@ -85,7 +85,7 @@
 }
 
 __noreturn void __libc_init(void* raw_args,
-                            void (*onexit)(void),
+                            void (*onexit)(void) __unused,
                             int (*slingshot)(int, char**, char**),
                             structors_array_t const * const structors) {
   KernelArgumentBlock args(raw_args);