meson: Use -fstrict-flex-arrays=3

Let's explicitly pass the value to -fstrict-flex-arrays. This does
not change behavior but it does (selfishly) make my error not bug
out with an error saying -fstrict-flex-arrays does not exist.

(cherry picked from commit ad723ca3e5bd41d2d884760375534910bb55d9b3)
(cherry picked from commit 2925fc2c6f4b13a2f098912fa3d44ad31e9f2cf0)

meson.build

@@ -381,7 +381,7 @@ possible_common_cc_flags = [
         '-fno-common',
         '-fstack-protector',
         '-fstack-protector-strong',
-        '-fstrict-flex-arrays',
+        '-fstrict-flex-arrays=3',
         '--param=ssp-buffer-size=4',
 ]