Make same crates available during probe as in crate root

Author: dtolnay

src/probe/proc_macro_span_file.rs

@@ -4,6 +4,7 @@
 
 extern crate alloc;
 extern crate proc_macro;
+extern crate std;
 
 use alloc::string::String;
 use proc_macro::Span;

src/probe/proc_macro_span_location.rs

@@ -2,7 +2,9 @@
 
 #![cfg_attr(procmacro2_build_probe, no_std)]
 
+extern crate alloc;
 extern crate proc_macro;
+extern crate std;
 
 use proc_macro::Span;