LibC: Mark compilation-unit-only functions as static

This enables a nice warning in case a function becomes dead code.
For example with the unused function malloc_good_size() :^)

I found these places by using -Wmissing-declarations.

The Kernel still shows these issues, which I think are false-positives,
but don't want to touch:
- Libraries/LibC/crt0.cpp:41:5: int _start(int, char**, char**)
	Not sure how to handle this.
- Libraries/LibC/cxxabi.cpp:48:5: int __cxa_atexit(AtExitFunction, void*, void*)
- Libraries/LibC/cxxabi.cpp:58:6: void __cxa_finalize(void*)
	Not sure how to tell the compiler that the compiler is already using them.
- Libraries/LibC/libcinit.cpp:36:6: void __libc_init()
- Libraries/LibC/libcinit.cpp:55:19: void __stack_chk_fail()
- Libraries/LibC/malloc.cpp:430:6: void __malloc_init()
- Libraries/LibC/stdio.cpp:562:6: void __stdio_init()
	These are ninja-imported by other LibC functions.
	Maybe we should have some kind of "internals.h" header.

Libraries/LibC/malloc.cpp

@@ -182,15 +182,6 @@ static BigAllocator* big_allocator_for_size(size_t size)
 
 extern "C" {
 
-size_t malloc_good_size(size_t size)
-{
-    for (size_t i = 0; size_classes[i]; ++i) {
-        if (size < size_classes[i])
-            return size_classes[i];
-    }
-    return PAGE_ROUND_UP(size);
-}
-
 static void* os_alloc(size_t size, const char* name)
 {
     auto* ptr = serenity_mmap(nullptr, size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE | MAP_PURGEABLE, 0, 0, block_size, name);