Fix bosses flicking across the screen when hurt

The Wii U's wacky PowerPC architecture highlighted a cool
platform-dependency: the boss rendering code only properly works on
platforms where `char` is signed by default.

Granted, it's possible Pixel explicitly marked this variable as
signed. Who knows. Either way, CSE2 didn't, and now that's been
fixed.

src/Boss.cpp

@@ -35,7 +35,7 @@ void InitBossChar(int code)
 
 void PutBossChar(int fx, int fy)
 {
-	char a = 0;
+	signed char a = 0;
 	int b;
 
 	int side;