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/scanf.cpp

@@ -61,7 +61,7 @@ static const char* determine_base(const char* p, int& base)
 static int _atob(unsigned long* vp, const char* p, int base)
 {
     unsigned long value, v1, v2;
-    const char *q;
+    const char* q;
     char tmp[20];
     int digit;
 
@@ -106,7 +106,7 @@ static int _atob(unsigned long* vp, const char* p, int base)
     return 1;
 }
 
-int atob(unsigned int* vp, const char* p, int base)
+static int atob(unsigned int* vp, const char* p, int base)
 {
     unsigned long v;