server/meson.build: support relative path for prebuilt_server
server/meson.build: support relative path for prebuilt_server

If we don't do this trick, the prebuilt_server will be
../server/[the_user_defined_path]. In general, we will not give an relative path
based on build directory, which leads to wrong prebuilt_server path.

The building error:

ninja: error: '../scrcpy-server-v1.7.jar', needed by
'server/scrcpy-server.jar', missing and no known rule to make it

Signed-off-by: Yu-Chen Lin <4520184cbb4d42154975f8b8b0138505e78cc728@gmail.com>

Genymobile/scrcpy,Genymobile/scrcpy,Genymobile/scrcpy