utils.c: Fix implicit declaration in functions ´strntolower´ and ´strntoupper´

See,
utils.c: In function ‘strntolower’:
utils.c:786:24: warning: implicit declaration of function ‘tolower’ [-Wimplicit-function-declaration]
                 *ptr = tolower(*ptr);
                        ^~~~~~~
utils.c: In function ‘strntoupper’:
utils.c:796:24: warning: implicit declaration of function ‘toupper’ [-Wimplicit-function-declaration]
                 *ptr = toupper(*ptr);
                        ^~~~~~~

plugins/utils.c

@@ -27,6 +27,7 @@
 #include "utils_base.h"
 #include <stdarg.h>
 #include <limits.h>
+#include <ctype.h>
 
 #include <arpa/inet.h>