Add missing stdint.h includes in unit tests files

My mingw compiler/headers (mingw-w64 10.0.0 on macOS) seem to be more
pendantic than the one that comes with Ubuntu 22.04 (github actions) or
any of the other platforms including msvc/normal windows header.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20230208001819.244694-5-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26182.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit e80720ef93)

tests/unit_tests/example_test/test.c

@@ -4,6 +4,7 @@
 #include <stdarg.h>
 #include <string.h>
 #include <setjmp.h>
+#include <stdint.h>
 #include <cmocka.h>
 
 static int

tests/unit_tests/example_test/test2.c

@@ -4,6 +4,7 @@
 #include <stdarg.h>
 #include <string.h>
 #include <setjmp.h>
+#include <stdint.h>
 #include <cmocka.h>
 
 

tests/unit_tests/openvpn/mock_msg.c

@@ -32,6 +32,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <setjmp.h>
+#include <stdint.h>
 #include <cmocka.h>