meson: allow checking for null pointers even if they're supposed to be non-null

../src/c11/impl/time.c: In function 'timespec_get':
    ../src/c11/impl/time.c:71:8: error: 'nonnull' argument 'ts' compared to NULL [-Werror=nonnull-compare]
       71 |     if (!ts)
          |        ^
    cc1: all warnings being treated as errors

Signed-off-by: Eric Engestrom <eric@igalia.com>
Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Acked-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21558>

meson.build

@@ -995,6 +995,7 @@ else
     '-Wmisleading-indentation',
     '-Wno-missing-field-initializers',
     '-Wno-format-truncation',
+    '-Wno-nonnull-compare',
     '-fno-math-errno',
     '-fno-trapping-math',
     '-Qunused-arguments',