tests/tcg: add message to _Static_assert in test-avx

In preparation for enabling clang and avoiding:

  error: '_Static_assert' with no message is a C2x extension [-Werror,-Wc2x-extensions]

let us just add the message to silence the warning.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250304222439.2035603-15-alex.bennee@linaro.org>
diff --git a/tests/tcg/i386/test-avx.c b/tests/tcg/i386/test-avx.c
index 230e6d8..80fe363 100644
--- a/tests/tcg/i386/test-avx.c
+++ b/tests/tcg/i386/test-avx.c
@@ -244,7 +244,7 @@
                0x0000003afffffff0ull, 0x000000000000000eull};
 
 v4di gather_mem[0x20];
-_Static_assert(sizeof(gather_mem) == 1024);
+_Static_assert(sizeof(gather_mem) == 1024, "gather_mem not expected size");
 
 void init_f16reg(v4di *r)
 {