shared/util.c: assert_cc() can only be used inside functions

shared/util.c: assert_cc() can only be used inside functions

shared/macro.h has two versions of assert_cc, one that uses gcc
_Static_assert(), which requires recent enough gcc versions, and one
that uses a fake array to trigger a build error. The latter can only
work inside functions, so assert_cc() should only be used inside
functions.

Fixes the following build failure when building kmod with old gcc
versions such as gcc 4.3.x:

shared/util.c:52: error: expected identifier or '(' before 'do'
shared/util.c:52: error: expected identifier or '(' before 'while'

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>