Correct warning if used -Wextra

argsparse.h

@@ -144,7 +144,7 @@ static unsigned char __compile_and_equals_sflag(char* flags, char* str) {
 }
 
 static char* malloc_str(char* str) {
-    char* ptr = malloc((strlen(str)) * sizeof(char) + sizeof(char));
+    char* ptr = (char*)malloc((strlen(str)) * sizeof(char) + sizeof(char));
     strcpy(ptr, str);
     return ptr;
 }