kconfig: Fix realloc usage()

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/kconfig/symbol.c

@@ -878,7 +878,7 @@ const char *sym_expand_string_value(const char *in)
 		newlen = strlen(res) + strlen(symval) + strlen(src);
 		if (newlen > reslen) {
 			reslen = newlen;
-			realloc(res, reslen);
+			res = realloc(res, reslen);
 		}
 
 		strcat(res, symval);