meson: add -Wgnu-pointer-arith to _trial_msvc

In theory, this will prevent the warning in the future
in common code (which uses c_msvc_compat_args).

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38752>

meson.build

@@ -1255,7 +1255,12 @@ else
   # Check for C and C++ arguments for MSVC compatibility. These are only used
   # in parts of the mesa code base that need to compile with MSVC, mainly
   # common code
-  _trial_msvc = ['-Werror=pointer-arith', '-Werror=vla', '-Werror=gnu-empty-initializer']
+  _trial_msvc = [
+    '-Werror=pointer-arith',
+    '-Werror=vla',
+    '-Werror=gnu-empty-initializer',
+    '-Wgnu-pointer-arith',
+  ]
   c_msvc_compat_args += cc.get_supported_arguments(_trial_msvc)
   cpp_msvc_compat_args += cpp.get_supported_arguments(_trial_msvc)
 endif