build fix: added missing declaration of mhl_dir_plus_file

./configure --enable-charset && make
    charsets.c: In function 'load_codepages_list':
    charsets.c:51: warning: implicit declaration of function 'mhl_str_dir_plus_file'
    charsets.c:51: warning: assignment makes pointer from integer without a cast

Signed-off-by: Sergei Trofimovich <slyfox@inbox.ru>

src/charsets.c

@@ -27,6 +27,8 @@
 
 #include <iconv.h>
 
+#include <mhl/string.h>
+
 #include "global.h"
 #include "charsets.h"