Mark input argument to hextorgb const

[cgzones]

tests/image_tests.c: In function ‘hextorgb_can_convert_fn’:
tests/image_tests.c:738:18: warning: passing argument 1 of ‘hextorgb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  738 |         hextorgb(000000, rgb);
      |                  ^~~~~~~~
In file included from tests/image_tests.c:7:
./src/image_support.h:18:21: note: expected ‘char *’ but argument is of type ‘const char *’
   18 | void hextorgb(char *input, int *rgb);
      |               ~~~~~~^~~~~

[vergoh]

Thanks