[RPM] Add virtual conf package

[RPM] Add virtual conf package

Add virtual conf package to resolve build dependency issue as suggested
in https://github.com/nnstreamer/api/pull/21#issuecomment-804599118

With this patch, it is now possible to install devel package with
`/etc/nntrainer.ini` or the packager choose to use `test-devel` when
building.

See also nnstreamer/nntrainer#1034

**Evaluation Method**
Cross building with nntrainer package

**Self evaluation:**
1. Build test: [X]Passed [ ]Failed [ ]Skipped
2. Run test: [X]Passed [ ]Failed [ ]Skipped

Cc: Myungjoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Jihoon Lee <jhoon.it.lee@samsung.com>