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/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);