print: fix function definition

lib/print.c

@@ -30,7 +30,7 @@
 
 uint8_t print_error;
 
-size_t puts(char* str)
+size_t puts(const char* str)
 {
 	size_t printed = 0; // number of characters printed
 	while (*str) { // go until end of string (\0 string termination character)

lib/print.h

@@ -54,7 +54,7 @@ size_t putc(char c);
  *  @note uses putc to output characters
  *  @note does not require print functions (and is faster/simpler)
  */
-size_t puts(char* str);
+size_t puts(const char* str);
 /** print format string on user output
  *  @param[in] format format string to be printed
  *  @param[in] ... arguments referenced by format string to be printed