Kernel: Add dbgputch() to kstdio.h

This adds a dbgputch() alongside dbgputstr() in kstdio.h. The function
already existed as the static function debugger_out(). It has now been
exposed to users of kstdio.h.

Kernel/kprintf.cpp

@@ -145,7 +145,7 @@ int snprintf(char* buffer, size_t size, const char* fmt, ...)
  return ret;
 }
 
-static void debugger_out(char ch)
+extern "C" void dbgputch(char ch)
 {
  if (serial_debug)
  serial_putch(ch);
@@ -158,7 +158,7 @@ extern "C" void dbgputstr(const char* characters, size_t length)
  return;
  ScopedSpinLock lock(s_log_lock);
  for (size_t i = 0; i < length; ++i)
- debugger_out(characters[i]);
+ dbgputch(characters[i]);
 }
 
 extern "C" void kernelputstr(const char* characters, size_t length)

Kernel/kstdio.h

@@ -9,6 +9,7 @@
 #include <AK/Types.h>
 
 extern "C" {
+void dbgputch(char);
 void dbgputstr(const char*, size_t);
 void kernelputstr(const char*, size_t);
 void kernelcriticalputstr(const char*, size_t);