Fix declaration of pubkeys in test_provider.c in MSVC builds

Fix declaration of pubkeys in test_provider.c in MSVC builds

  Error: test_provider.c(74): error C2099: initializer is not a constant

Fix this issue by making the const char* to const char[]. This is probably
of one the weird array decay corner cases

I could not find another/better way around this issue.

This error only occurs when building unit tests with windows which our
normal build system does not do but my out of tree cmake build script
tries and fails.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20220824165718.102002-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25102.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>