build: fix mingw build of vbox

build: fix mingw build of vbox

More fallout from commit 7c9a2d88 dropping too many headers.  Fixes:

In file included from ../../src/vbox/vbox_glue.c:26:0:
../../src/vbox/vbox_MSCOMGlue.c: In function 'vboxLookupVersionInRegistry':
../../src/vbox/vbox_MSCOMGlue.c:435:5: error: implicit declaration of function 'virParseVersionString' [-Werror=implicit-function-declaration]
...
../../src/vbox/vbox_driver.c: In function 'vboxConnectOpen':
../../src/vbox/vbox_driver.c:147:5: error: implicit declaration of function 'getuid' [-Werror=implicit-function-declaration]
../../src/vbox/vbox_driver.c:147:5: error: nested extern declaration of 'getuid' [-Werror=nested-externs]

* src/vbox/vbox_MSCOMGlue.c (includes): Add missing includes.
* src/vbox/vbox_driver.c (includes): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>