i965: Fix build error with clang.

i965: Fix build error with clang.

Technically, variable sized arrays are a required feature of C99,
redacted to be optional in C11, and not actually part of C++ whatsoever.

Gcc allows using them in C++ unless you specify -pedantic, and Clang
appears to allow them for simple/POD types.

exec_list is arguably POD, since it doesn't have virtual methods, but I
can see why Clang would be like "meh, it's a C++ struct, say no", seeing as
it's meant to support C99.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=58970
Reviewed-by: Matt Turner <mattst88@gmail.com>