genksyms: Ignore module scoped _Static_assert()

The C11 _Static_assert() keyword may be used at module scope, and we
need to teach genksyms about it to not abort with an error. We currently
have a growing number of static_assert() (but also direct usage of
_Static_assert()) users at module scope:

	git grep -E '^_Static_assert\(|^static_assert\(' | grep -v '^tools' | wc -l
	135

More recently, when enabling CONFIG_MODVERSIONS with CONFIG_KCSAN, we
observe a number of warnings:

	WARNING: modpost: EXPORT symbol "<..all kcsan symbols..>" [vmlinux] [...]

When running a preprocessed source through 'genksyms -w' a number of
syntax errors point at usage of static_assert()s. In the case of
kernel/kcsan/encoding.h, new static_assert()s had been introduced which
used expressions that appear to cause genksyms to not even be able to
recover from the syntax error gracefully (as it appears was the case
previously).

Therefore, make genksyms ignore all _Static_assert() and the contained
expression. With the fix, usage of _Static_assert() no longer cause
"syntax error" all over the kernel, and the above modpost warnings for
KCSAN are gone, too.

Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>