libutf: Change return type of utftorunestr to size_t

It returns the size of the rune array, so size_t is the right type
to use here.

utf.h

@@ -59,7 +59,7 @@ int isxdigitrune(Rune);
 Rune tolowerrune(Rune);
 Rune toupperrune(Rune);
 
-int utftorunestr(const char*, Rune *);
+size_t utftorunestr(const char *, Rune *);
 
 int fgetrune(Rune *, FILE *);
 int efgetrune(Rune *, FILE *, const char *);