[Maintenance] [th02] graph_putsa_fx(): Explicitly declare as `extern "C"`

Needed for a constness overloading hack later. A certain single place
in TH02 will need the `str` parameter to not be `const` for code
generation reasons, but we don't want to force every call site into
passing non-`const` pointers for such a silly reason.

Part of P0278, funded by Yanga.

th01/hardware/grppsafx.h

@@ -31,6 +31,9 @@
 
 	// Puts the given [str] onto the graphics RAM at the given position,
 	// with the given graphics color and effect.
+	#if (GAME == 2)
+		extern "C"
+	#endif
 	void DEFCONV graph_putsa_fx(
 		screen_x_t left, vram_y_t top, int16_t col_and_fx, const shiftjis_t *str
 	);

th02/op_01.cpp

@@ -13,9 +13,7 @@
 #include "libs/kaja/kaja.h"
 #include "th01/rank.h"
 #include "th01/math/clamp.hpp"
-extern "C" {
 #include "th01/hardware/grppsafx.h"
-}
 #include "th02/common.h"
 #include "th02/resident.hpp"
 #include "th02/hardware/frmdelay.h"

th02/op_05.cpp

@@ -10,12 +10,10 @@
 #include "master.hpp"
 #include "shiftjis.hpp"
 #include "th01/math/clamp.hpp"
-extern "C" {
 #include "th01/hardware/grppsafx.h"
 #include "th02/v_colors.hpp"
 #include "th02/common.h"
 #include "th02/resident.hpp"
-}
 #include "th02/hardware/frmdelay.h"
 #include "th02/hardware/grp_rect.h"
 #include "th02/hardware/input.hpp"