lib/charset: fix compile warnings

This commit fixes the following compile warnings
for the documentation.

./include/charset.h:276: warning: Function parameter or member 'size' not described in 'u16_strlcat'
./include/charset.h:276: warning: Excess function parameter 'count' description in 'u16_strlcat'
Signed-off-by: NMasahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: NHeinrich Schuchardt <heinrich.schuchardt@canonical.com>

include/charset.h

@@ -273,7 +273,7 @@ u16 *u16_strdup(const void *src);
  * Return:		required size including trailing 0x0000 in u16 words
  *			If return value >= count, truncation occurred.
  */
-size_t u16_strlcat(u16 *dest, const u16 *src, size_t size);
+size_t u16_strlcat(u16 *dest, const u16 *src, size_t count);
 
 /**
  * utf16_to_utf8() - Convert an utf16 string to utf8