bugfix unmatched dlls in vcpkg_copy_pdbs

[silverqx]

Fix if condition to check unmatched dlls.

Currently, the unmatched_dlls_length variable is used, but this variable is not defined anywhere, it should be dlls_without_matching_pdbs.

• What does your PR fix?

Currently, if you use vcpkg_copy_pdbs(), then following condition is always true because unmatched_dlls_length doesn't exist and because of that the vcpkg output contains this error message everytime:

CMake Warning at scripts/cmake/vcpkg_copy_pdbs.cmake:70 (message):
  Could not find a matching pdb file for:


Call Stack (most recent call first):
  ports/hello-world-library/portfile.cmake:57 (vcpkg_copy_pdbs)
  scripts/ports.cmake:141 (include)

[cenit]

duplicate of #19369 ?

[silverqx]

It is @cenit, so I have approved #19369 and closing this PR.