go.mod, tools: use Go 1.24 tool dependencies

Use the tool go.mod directive introduced in Go 1.24 [^1] to specify
the ebpf2go tool that is needed during build. This allows to drop the
tools/tools.go which was used so far to pull in this dependency.

[^1]: https://go.dev/doc/modules/managing-dependencies#tools

Signed-off-by: Tobias Klauser <tobias@cilium.io>

Author: tklauser

go.mod

@@ -31,3 +31,5 @@ require (
 	github.com/rivo/uniseg v0.4.7 // indirect
 	rsc.io/c2go v0.0.0-20170620140410-520c22818a08 // indirect
 )
+
+tool github.com/cilium/ebpf/cmd/bpf2go