(.gitignore): Do not ignore debug/

In PR operator-framework#888, there's vendoring of the cel-go package, one of which
is named "debug" (github.com/google/cel-go/common/debug). Due to
the .gitignore file ignoring all [Dd]bug folders, the vendored
package isn't being checked in by git, causing the build to fail.
This PR removes the instruction to ignore [Dd]bug/ folders.

Signed-off-by: Anik Bhattacharjee <anikbhattacharya93@gmail.com>

.gitignore

@@ -179,7 +179,6 @@ tags
 *.userprefs
 
 # Build results
-[Dd]ebug/
 [Dd]ebugPublic/
 x64/
 x86/