Add clang-15+ prerequisite to README (#8203)

Summary:
Multiple issues (#8112, #8195) have been created related to build errors because the build isn't using `clang-15` or greater.

Pull Request resolved: #8203

Reviewed By: lnyng

Differential Revision: D48659974

Pulled By: brianc118

fbshipit-source-id: b7633a2bc1f2f645a0cb3a43f4647ee59e6464a6

docs/building.md

@@ -5,7 +5,7 @@ Several dependencies need to be installed before we can build.
 * libz (dynamically linked)
 * libelf (dynamically linked)
 * libncursesw (dynamically linked)
-* clang (for building BPF program at build time)
+* clang-15+ (for building BPF program at build time)
 * rustfmt (used by libbpf-cargo for bpf skeleton code generation)
 
 ## Install build dependencies
@@ -27,6 +27,11 @@ rustup component add rustfmt
 
 # Building
 
+Make sure clang-15 is installed and exported.
+```shell
+export CLANG=clang-15
+```
+
 Below's UI is quite laggy in debug builds. We recommend always building in
 release mode.