Constify sbbs_t::center()

src/sbbs3/con_out.cpp

@@ -787,7 +787,7 @@ void sbbs_t::inc_row(int count)
 	}
 }
 
-void sbbs_t::center(char *instr, unsigned int columns)
+void sbbs_t::center(const char *instr, unsigned int columns)
 {
 	char str[256];
 	size_t len;

src/sbbs3/sbbs.h

@@ -759,7 +759,7 @@ public:
 	int		outchar(enum unicode_codepoint, const char* cp437_fallback = NULL);
 	void	inc_row(int count);
 	void	inc_column(int count);
-	void	center(char *str, unsigned int columns = 0);
+	void	center(const char *str, unsigned int columns = 0);
 	void	wide(const char*);
 	void	clearscreen(long term);
 	void	clearline(void);