libkirk: let C++ callers include its headers directly

kirk_engine.h and amctrl.h guard their declarations, but AES.h and SHA1.h
never did, and kirk_engine.h includes them from outside its own guard. So the
AES_* and SHA1* functions got C++ linkage in any C++ file that reached them
through there, and only linked for callers that happened to wrap the whole
header in an extern "C" of their own. Nothing had called AES_* from C++
before, so it stayed hidden until something did.

Guarding the two headers instead lets every caller include them plainly, and
the wrappers scattered around the tree come out. Both are pure declarations
over kirk_common.h's typedefs with no system headers behind them, so there's
nothing in there that shouldn't be wrapped.

kirk_engine.h also uses size_t without including anything that defines it,
which only held together because its includers happened to have it already.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Core/FileSystems/BlockDevices.cpp

@@ -35,12 +35,9 @@
 #include "Core/Util/PathUtil.h"
 #include "libchdr/chd.h"
 
-extern "C"
-{
 #include "zlib.h"
 #include "ext/libkirk/amctrl.h"
 #include "ext/libkirk/kirk_engine.h"
-};
 
 static u16 ReadLE16(const u8 *ptr) {
 	return ptr[0] | (ptr[1] << 8);