Constify results from stdlib functions

Constify results from stdlib functions

This fixes recent warnings:

../src/st/croco/cr-rgb.c: In function ‘cr_rgb_set_from_name’:
../src/st/croco/cr-rgb.c:405:16: warning: assignment discards ‘const’
qualifier from pointer target type [-Wdiscarded-qualifiers]
  405 |         result = bsearch (a_color_name,
      |                ^
[150/294] Compiling C object src/libshell-18.so.p/shell-util.c.o
../src/shell-util.c: In function ‘shell_util_translate_time_string’:
../src/shell-util.c:165:7: warning: assignment discards ‘const’
qualifier from pointer target type [-Wdiscarded-qualifiers]
  165 |   sep = strchr (str, '\004');
      |       ^

which seem to originate from:

https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=cd748a63ab1a7ae846175c532a3daab341c62690;hp=64b6813338d836ad1b61969a153192fb739f8722

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4139>