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/termcap.cpp

@@ -24,8 +24,8 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <AK/String.h>
 #include <AK/HashMap.h>
+#include <AK/String.h>
 #include <assert.h>
 #include <stdio.h>
 #include <string.h>
@@ -54,7 +54,7 @@ int tgetent(char* bp, const char* name)
 
 static HashMap<String, const char*>* caps = nullptr;
 
-void ensure_caps()
+static void ensure_caps()
 {
     if (caps)
         return;