build: fix install from any directory with Meson 0.55

build: fix install from any directory with Meson 0.55

Install command on Windows for Meson >= 0.55.0 referenced the script
by a plain string, assuming the build directory to be directly under
the source tree root.
This resulted in an error when the assumption did not hold:

    c:\python\python.exe: can't open file
    '../buildtools/symlink-drivers-solibs.py':
    [Errno 2] No such file or directory

Use files() to make a valid script path for any build directory.

Fixes: cd27047dbee1 ("build: support drivers symlink on Windows")
Cc: stable@dpdk.org
Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Nick Connolly <nick.connolly@mayadata.io>