Fix compiler warning - using "const" twice for CHARSET_INFO.

CHARSET_INFO is already typedef'ed as "const" strúcture, thus "const CHARSET_INFO"  generates  multiple type modifier warning.

include/m_ctype.h

@@ -620,9 +620,9 @@ my_bool my_charset_is_ascii_compatible(CHARSET_INFO *cs);
 extern size_t my_vsnprintf_ex(CHARSET_INFO *cs, char *to, size_t n,
                               const char* fmt, va_list ap);
 
-uint32 my_convert(char *to, uint32 to_length, const CHARSET_INFO *to_cs,
+uint32 my_convert(char *to, uint32 to_length, CHARSET_INFO *to_cs,
                   const char *from, uint32 from_length,
-                  const CHARSET_INFO *from_cs, uint *errors);
+                  CHARSET_INFO *from_cs, uint *errors);
 
 #define	_MY_U	01	/* Upper case */
 #define	_MY_L	02	/* Lower case */

strings/ctype.c

@@ -447,9 +447,9 @@ my_charset_is_ascii_compatible(CHARSET_INFO *cs)
 
 static uint32
 my_convert_internal(char *to, uint32 to_length,
-                    const CHARSET_INFO *to_cs,
+                    CHARSET_INFO *to_cs,
                     const char *from, uint32 from_length,
-                    const CHARSET_INFO *from_cs, uint *errors)
+                    CHARSET_INFO *from_cs, uint *errors)
 {
   int         cnvres;
   my_wc_t     wc;
@@ -517,9 +517,9 @@ outp:
 */
 
 uint32
-my_convert(char *to, uint32 to_length, const CHARSET_INFO *to_cs,
+my_convert(char *to, uint32 to_length, CHARSET_INFO *to_cs,
            const char *from, uint32 from_length,
-           const CHARSET_INFO *from_cs, uint *errors)
+           CHARSET_INFO *from_cs, uint *errors)
 {
   uint32 length, length2;
   /*