tests: virpcimock: Always declare __open_2

tests: virpcimock: Always declare __open_2

In some cases e.g. with clang on fedora 30 __open2 isn't even declared
which results in the following build error:

/home/pipo/libvirt/tests/virpcimock.c:939:1: error: no previous prototype for function
      '__open_2' [-Werror,-Wmissing-prototypes]
__open_2(const char *path, int flags)

Add a separate declaration to appease the compiler.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>