cmake: Fix files directly in data directory breaking project generation

[gxalpha]

Description

Fixes files directly in the data directory breaking the cmake project generation. @PatTheMav suggested this fix, which also brings this function in line with the one used in obs-studio. The template may get rewritten again at some point, but until then I think it's useful to fix the problem.

Motivation and Context

Having a file directly in the data directory would break the cmake project generation:

CMake Error at cmake/ObsPluginHelpers.cmake:469 (string):
  string sub-command REPLACE requires at least four arguments.
Call Stack (most recent call first):
  cmake/ObsPluginHelpers.cmake:456 (install_bundle_resources)
  CMakeLists.txt:95 (setup_plugin_target)

How Has This Been Tested?

Tested that this fix works with a plugin I made where I ran into the problem.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.