meson: disable -Wnonnull-compare

This gets enabled by default in gcc-14 and complains everywhere where we
use assert() on an expression that is always true (i.e. using
`int x[static 2]` in function declaration, etc.):

[153/2414] Compiling C object src/basic/libbasic.a.p/fs-util.c.o
In file included from ../src/basic/macro.h:13,
                 from ../src/basic/alloc-util.h:10,
                 from ../src/basic/fs-util.c:11:
../src/basic/fd-util.h: In function ‘format_proc_fd_path’:
../src/fundamental/macro-fundamental.h:74:41: warning: ‘nonnull’ argument ‘buf’ compared to NULL [-Wnonnull-compare]
   74 | #define _unlikely_(x) (__builtin_expect(!!(x), 0))
      |                                         ^~~~~
../src/basic/macro.h:150:21: note: in expansion of macro ‘_unlikely_’
  150 |                 if (_unlikely_(!(expr)))                                \
      |                     ^~~~~~~~~~
../src/basic/macro.h:167:22: note: in expansion of macro ‘assert_message_se’
  167 | #define assert(expr) assert_message_se(expr, #expr)
      |                      ^~~~~~~~~~~~~~~~~
../src/basic/fd-util.h:129:9: note: in expansion of macro ‘assert’
  129 |         assert(buf);
      |         ^~~~~~

Disabling this selectively only for asserts is a bit painful, since the
option is not available in all compilers, and it'd need to be handled in
the EFI stuff as well.

(cherry picked from commit b59bce308d)
(cherry picked from commit 08e99f781e)
(cherry picked from commit d8afb669da)

meson.build

@@ -351,6 +351,7 @@ endif
 basic_disabled_warnings = [
         '-Wno-missing-field-initializers',
         '-Wno-unused-parameter',
+        '-Wno-nonnull-compare',
 ]
 
 possible_common_cc_flags = [