modpost: Declare extra_warn with unused attribute

A recent strengthening of -Wunused-but-set-variable (enabled with -Wall)
in clang under a new subwarning, -Wunused-but-set-global, points out an
unused static global variable in scripts/mod/modpost.c:

  scripts/mod/modpost.c:59:13: error: variable 'extra_warn' set but not used [-Werror,-Wunused-but-set-global]
     59 | static bool extra_warn;
        |             ^

This variable has been unused since commit 6c6c1fc09d ("modpost:
require a MODULE_DESCRIPTION()") but that is expected, as there are
currently no extra warnings at W=1 right now. Declare the variable with
the unused attribute to make it clear to the compiler that this variable
may be unused.

Cc: stable@vger.kernel.org
Fixes: 6c6c1fc09d ("modpost: require a MODULE_DESCRIPTION()")
Link: https://patch.msgid.link/20260325-modpost-extra_warn-unused-but-set-global-v1-1-2e84003b7e81@kernel.org
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

scripts/mod/modpost.c

@@ -56,7 +56,7 @@ static bool allow_missing_ns_imports;
 
 static bool error_occurred;
 
-static bool extra_warn;
+static bool extra_warn __attribute__((unused));
 
 bool target_is_big_endian;
 bool host_is_big_endian;