doc: mention python3-distutils dependency in BUILDING.md

Ran into #30189, solved by this comment:
#30189 (comment)

Refs: #30189
PR-URL: #33174
Reviewed-By: Ben Noordhuis <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: Christian Clauss <[email protected]>

Author: osher

BUILDING.md

@@ -249,6 +249,8 @@ Installation via Linux package manager can be achieved with:
 
 FreeBSD and OpenBSD users may also need to install `libexecinfo`.
 
+Python 3 users may also need to install `python3-distutils`.
+
 #### macOS prerequisites
 
 * Xcode Command Line Tools >= 10 for macOS