configure: Add missing POSIX-required space

configure: Add missing POSIX-required space

In commit 7d7dbf9dc15be6e1 we added a line to the configure script
which is not valid POSIX shell syntax, because it is missing a space
after a '!' character. shellcheck diagnoses this:

if !(GIT="$git" "$source_path/scripts/git-submodule.sh" "$git_submodules_action" "$git_submodules"); then
    ^-- SC1035: You are missing a required space after the !.

and the OpenBSD shell will not correctly handle this without the space.

Fixes: 7d7dbf9dc15be6e1 ("configure: replace --enable/disable-git-update with --with-git-submodules")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Tested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: 20220720152631.450903-2-peter.maydell@linaro.org