fix(build): Disable -Wchar-subscripts warnings

Disable these warnings as they don't really help in our case:

    In file included from ../epicardium/modules/config.c:8:
    ../epicardium/modules/config.c: In function 'trim':
    ../epicardium/modules/config.c:133:28: warning: array subscript has type 'char' [-Wchar-subscripts]
      133 |  while (*start && !isgraph(*start))
          |                            ^~~~~~
    ../epicardium/modules/config.c:138:27: warning: array subscript has type 'char' [-Wchar-subscripts]
      138 |   while (*end && !isgraph(*end))
          |                           ^~~~
    ../epicardium/modules/config.c: In function 'epic_config_get_string':
    ../epicardium/modules/config.c:335:26: warning: array subscript has type 'char' [-Wchar-subscripts]
      335 |  while (*end && !iscntrl(*end))
          |                          ^~~~
    ../epicardium/modules/config.c: In function 'epic_config_set_string':
    ../epicardium/modules/config.c:519:28: warning: array subscript has type 'char' [-Wchar-subscripts]
      519 |   while (*end && (!iscntrl(*end) || isblank(*end)))
          |

Signed-off-by: Rahix <rahix@rahix.de>

meson.build

@@ -20,6 +20,7 @@ assert(
 add_global_arguments(
   '-Wno-unused-parameter',
   '-Wno-old-style-declaration',
+  '-Wno-char-subscripts',
   meson.get_cross_property('target_defs'),
   language: 'c',
 )