[libc][NFC] mark vprintf functions as inline

[michaelrj]

Summary

The functions are in a header, and so must be marked inline to avoid
symbol conflicts.

libc/src/stdio/printf_core/vfprintf_internal.h

LIBC_INLINE int fwrite_unlocked(const void *ptr, size_t size, size_t nmemb,
                                 ::FILE *f) {
   return ::fwrite_unlocked(ptr, size, nmemb, f);
 }
 #endif // LIBC_COPT_PRINTF_USE_SYSTEM_FILE
 } // namespace internal
 
 namespace printf_core {
 
-int file_write_hook(cpp::string_view new_str, void *fp) {
+LIBC_INLINE int file_write_hook(cpp::string_view new_str, void *fp) {
   ::FILE *target_file = reinterpret_cast<::FILE *>(fp);
   // Write new_str to the target file. The logic preventing a zero-length write
   // is in the writer, so we don't check here.
   size_t written = internal::fwrite_unlocked(new_str.data(), sizeof(char),
                                              new_str.size(), target_file);
   if (written != new_str.size() || internal::ferror_unlocked(target_file))
     return FILE_WRITE_ERROR;
   return WRITE_OK;
 }
 
-int vfprintf_internal(::FILE *__restrict stream, const char *__restrict format,
+LIBC_INLINE int vfprintf_internal(::FILE *__restrict stream,
+                                  const char *__restrict format,
                                   internal::ArgList &args) {
   constexpr size_t BUFF_SIZE = 1024;
   char buffer[BUFF_SIZE];
   printf_core::WriteBuffer wb(buffer, BUFF_SIZE, &file_write_hook,
                               reinterpret_cast<void *>(stream));
   Writer writer(&wb);
   internal::flockfile(stream);
   int retval = printf_main(&writer, format, args);
   int flushval = wb.overflow_write("");